Just to clarify to myself and maybe someone else how relations on tables work I created this demo project Demo2TablesRelations.xpo.
There are two tables Orders2 and Collections with two relations defined on Orders2 that work as dynalinks for Collections.
When an user chooses an item from Collection all records in the last are filtered in regards with CollectionType previously chosen on Orders form.
To change or add new records to Collection one can open the appropriate form of this table by pressing Collection button on Orders form but the relation should be cleared in Init method of Collection form.
if (element.args() && element.args().record().TableId == TableNum(Orders2))
Finally, to stand up once, I added the filter option by CollectionType.