For the combo box, if you set the Source Entity it will use all the records present on that Entity.

For example, in this case the combo box will show all the users:



To limit the results you need to add an aggregate to the preparation to return only the records you want and then set the "Source Record List" attribute on the combo box to use that aggregate.

In this case, instead of showing all the users, the combo box will only show the result of the "GetUserById" aggregate:



Hope this helps.