What is the difference between claiming a shift and applying for a shift?

First of all: outside the flexpool you do not claim a shift. You apply for a shift. This means that you do not immediately get a shift, but that you are placed in a kind of 'queue' until the employer accepts or rejects your application.

As soon as your application is accepted, you will receive an e-mail.

NOTE: You will not receive a notification or email if your application is rejected. Please keep a close eye on the app.

But - so how do I sign up? Check this article. For claiming, check this article.