meanwhile I found some time to implement your suggestion.
I implemented a "Processing Procedure Override" which populates a temporary table
collecting all check or uncheclk results.
This procedure fires for all rows which have a change in the check box (i.e. which have been selected or unselected).
This fires with a save button on the grid (not the form - but maybe I can do this later on. The form contains
2 tabs. One holding the base date of the main object, one holding the grid with the clickable relations)
But: As an After-Process I want to update the original Relation table with all changes.
That means, disableling or deleting all unchecked and inserting all checked relations.
For performance reasons it's not appropriate to do this as a "Post Processing Procedure" because
this fires for every changed row.
When I do it in a DA (button click event) this procedure is executed before the original grid override procedure,
which fills the temp table with the click/unclick data.
Question: Where or at which point could I call this post procedure right after the user clicked the save button.