Using on-hold or "waiting" contexts
You can set a context's status to "On Hold" with the inspector; actions assigned to an on-hold context are considered unavailable, and they block the progress of sequential projects. There are two main situations in which you might want to use on-hold contexts:
First, you might create one or more "waiting" contexts for keeping track of actions that you've delegated to other people. You can't actually do anything until you hear back from that person; all you can do is wait for them to finish it, and maybe nudge them about it every now and then. So an action like "get annotated pterodactyl brochure draft back from Dennis" might go in your "Waiting : Dennis" context.
Second, you might have some contexts that you don't expect to be available to you any time soon. You could put your "Frankfurt" context on hold when you're in London, or put your "Boss" context on hold until she comes back from vacation, and any assigned actions are marked unavailable. This helps you see which actions and projects aren't likely to make progress until your situation changes.