Who needs a paid seat, and who does not EN

Watching, commenting and downloading are free for everyone, forever. A seat is what one person needs to create in one workspace.

A paid seat is what one person needs to put new content into one shared workspace: to record into it, to upload into it, or to move a recording into it. Everything else people do with recordings - watching, commenting, downloading, editing, captions - needs no seat and costs nothing, for as many people as you like.

A seat belongs to one person in one workspace. A seat in one workspace gives you nothing in another, and Pro on your personal workspace gives you nothing in a shared one.

Who does not need a seat

Anyone who only watches. Viewers are free and unlimited, whether they are members of the workspace or people you sent a link to. They are never counted and never billed.
Anyone who comments, reacts or downloads. These need no seat and no role.
Anyone you invite to the workspace. Joining is free and is not limited by how many seats the workspace has. You can invite far more people than you have seats.
Anyone editing what is already there. Trimming, cutting, overlays, rendering, generating and fixing captions, transcripts, summaries and chapters are open to every member of the workspace, seated or not. Editing rewrites a recording that already exists, so it consumes no new capacity.
Admins and owners who never record there. Roles and seats are independent: an admin can hold no seat, and a plain member can hold one.
Anyone recording into their own personal workspace. Every account has one, and you never need a seat for it - see the free allowance below.

Who needs a seat

One seat, in that workspace, for each person who does any of the following in a shared workspace:

records into it;
uploads into it;
moves a recording into it from somewhere else.

A few consequences worth knowing:

The owner always holds one. While the workspace's plan is active, a seat is assigned to the owner and cannot be taken back, so a paid workspace always has at least one seat in use.
Seats can wait for someone. An admin can reserve a seat for a pending invitation, so it is already there the moment the person accepts. A reserved seat counts as used while it waits.
Two workspaces means two seats. If someone records into two shared workspaces, they need a seat in each.
On the free plan a shared workspace has no seats at all, so nobody can record into it - each person records into their own personal workspace instead.

If every purchased seat is already assigned, an admin either takes one back or buys more. See Give someone a paid seat, or take it back and Add seats now, and what proration costs. There is a limit on how many seats you can buy yourself, listed in Volume discounts and the seat cap.

Your personal workspace and the free allowance

You never need a seat in your own personal workspace. What you keep there counts towards your account's free allowance: 60 minutes of cloud recording time and 1 GB of cloud storage. Both belong to your account as a whole, not to a workspace, so creating more workspaces does not create more free capacity.

Buying a single seat upgrades your personal workspace. You cannot invite other people to a personal workspace, so from two seats up you are buying a shared workspace instead.

What a seat actually covers

Recordings you make in a workspace where you hold a seat are unlimited in time and storage, and do not count towards your free limits for as long as the seat and the workspace's plan last.
A seat does not make your free allowance bigger. It takes that workspace's recordings out of the count entirely - which is why the allowance looks untouched while you are seated.
Files you import into your asset library belong to your account rather than to a workspace, so they always count towards your free storage, seat or no seat.
Recording in Offline mode never needs a seat and never counts towards either limit, because nothing is stored with us.

When a seat ends

A seat can end because it was taken back, because it was given to someone else, or because the workspace's plan was cancelled, expired or went unpaid. Whichever it was:

You keep access to the workspace. You can still watch, comment, download, edit and delete what is there. Only creating new content in that workspace stops.
Your next recording goes to your personal workspace instead, because the workspace you lost the seat in is no longer a valid target.
The recordings you made there immediately start counting towards your account's free limits again. If that puts you over, nothing is deleted straight away - a 30-day grace period starts, and The 30-day grace period and the deletion order explains what happens next and how to stop it.

If you are removed from the workspace altogether, it works differently: your recordings and folders in that workspace move to the workspace owner, so they no longer count against your allowance, and the cards still credit you as the person who recorded them. See When someone leaves a workspace, or is removed.

Troubleshooting

"Read-only workspace - no paid seat" in the extension popup, or "You're not assigned a paid seat in this workspace." You are a member of that workspace without a seat. You can open and watch everything in it, but you cannot record into it. Ask an owner or admin of that workspace for a seat.
You cannot pick a workspace as your recording target. A workspace where you need a seat cannot be selected, and the extension falls back to your personal workspace. See Set where your next recording lands.
You have a seat elsewhere and still cannot record here. Seats do not travel between workspaces, and a Pro personal workspace does not cover a shared one. You need a seat in this particular workspace.
You hold a seat but recording is blocked anyway. A seat covers that workspace, not your whole account. If your personal workspace is over its free limits, that is a separate problem - see Which limit am I hitting: minutes or storage?.