Problem:

I am having an issue with CRED forms.

I want to perform some checks and changes to the submitted data before saving, as well as retrieve data from external fields not on the form.

Solution:

To perform validation on the form field, use the hook:

https://toolset.com/documentation/programmer-reference/cred-api/#cred_form_validate

To retrieve resources before submitting the form, use the hook:

https://toolset.com/documentation/programmer-reference/cred-api/#cred_before_save_data

To retrieve resources on the submit action, use the hook:

https://toolset.com/documentation/programmer-reference/cred-api/#cred_save_data

Problem:
I have a field Custom Fields Group ("Projekt-Felder") assigned to a CPT ("Projekte").
Inside this group i have – among many others – a Repeatable Group ("Downloads") with various fields.

I want this very repeatable group to be accessible by another CPT, but NOT all the other fields, which are in this Custom Fields Group.

Can i create a new Custom Fields Group, share this new group with "Projekte" and the new "CPT" and move the Repeatable Group into this new group? So that only this repeatable group is visible to the second, newly created CPT?
Solution:
What we can do is to use the previously built fields, but in a new RFG that you will need to create. You can do that by using this function, when adding a new field: https://snipboard.io/QNIjAM.jpg. Then, you'll see the already created fields, for example in my local installation: https://snipboard.io/vcsutU.jpg

Because of how repeatable field groups (RFG) are implemented from a technical perspective, it's not possible to use them in more than one post type or field group. There's no way to force that type of unsupported association programmatically. I think if it is very important to be able to reuse the View and fields, you could create one custom post type that acts as a container for the RFG. Keep the other less reusable fields, taxonomies, and relationships in separate post types.

Then you could use many-to-many post relationships to connect the RFG container post type to your other more specific post types. I'd have to know more about your project to be able to give you more concrete advice about what is the best way to achieve your end goals, and what is the most practical solution.