One space, two organizations' worth of membership rules
A shared channel is a communication space — a chat channel, a collaborative workspace — where members from two or more separate organizations coexist under one shared context, rather than the space belonging entirely to a single organization's own membership and access rules. This is deliberately useful for genuine cross-organization collaboration, and it also means the access-control model has to account for two different organizations' worth of members, permissions, and departure processes at once, rather than just one.
This differs from ordinary vendor access in a specific way: vendor access typically grants one external party defined access into a system the host organization fully controls, while a shared channel puts members from both organizations into a genuinely shared context that neither side unilaterally controls end to end.
Offboarding is the specific place this quietly breaks
The risk that's easy to overlook is offboarding: when someone leaves the partner organization, does anything on the host side actually notice and remove their access to the shared channel, or does that removal depend entirely on the partner organization telling the host side to do so? A shared channel design that assumes the other organization's own offboarding process will somehow propagate access changes automatically is assuming a synchronization that, in practice, usually doesn't exist without something explicit built to provide it — periodic review of external membership, or an integration that actually links the two organizations' identity systems together, rather than a design that simply hopes the partner remembers to mention it.