Projects can be broken down into parallel pieces of work, so you should always be able to pick up another part of the project rather than switch to a different project if you're blocked. As the other poster says, the goal is to not be blocked though. The leadership team need to support the delivery team to get the work done as a focused flow of items with as little blocking and waiting time as possible. If you're getting blocked, that's a signal that the org is failing.

Admittedly this is a little idealistic, but if it's not the goal then the reality is always far worse.

That's why I say the definition is "project" is key to this.

And what about investigating and fixing bugs? Bug reports don't come in on a schedule, and they're necessarily interrupt-driven unless you just don't care about your customers.