Hello Sebastian
I think that the main thing to consider is that a standard Catalog user cannot simply claim a request pending action. There are a few different ways that a request pending action can be actioned by someone other than the assignee, but none of these enable a Catalog user to claim a request pending action.
If a user has the appropriate role they can see the request pending action and then use the override functionality to approve or reject the request.
A user with the appropriate role can transfer the request pending action to another user. (see Catalog, Configuration, Options, Request Management Configuration, "Access Control: Proxy Request" setting)
A user with the appropriate role can define a delegate for their request pending actions. (see Catalog, Configuration, Options, Request Management Configuration, "Access Control: Delegate Request Pending Action" setting)
If you have any more questions, please ask here.
Steve