The criteria for whether or not the run-unit will be extended when an ADS dialog links to another one are documented here
Note that the DB-NAME is not considered.
CA-IDMS, all supported releases.
Changing the DB-NAME in ADS code does not impact whether or not a run-unit is extended.
So, if dialog A links to dialog B, and they both use the same subschema, but dialog B changes the DB-NAME before performing any DML, then the rununit will still be extended.
The fact that the DB-NAME changes would imply that dialog B is accessing a physically different database and therefore one might expect that the rununit cannot be extended.
However the ADS run-time system does not consider the DB-NAME change and so the run-unit is extended.