In ShotGrid, most entities are project entities, that belong to one project, and have a native (single) project field. E.g. Task, Version.
In the webhooks UI, if you select one or more projects for a webhook, the entity you choose must be one of these project entities, that belongs to a single project.
The UI will now warn you and prevent updating if you if you select a non-project entity type at the same time as having a project filter. (Previously the UI would allow you to set this up, but no webhook events would be sent.)
Unfortunately, entities like Person, which can belong to multiple projects at the same time, are also not allowed in a webhook with a project filter. We are sorry for the inconvenience of that! But it allows us to ensure that filtering of events for webhooks is as low-overhead as possible. (A multi-project entity would require querying more tables to determine if a webhook event should be generated.)