Naomi, when you start talking about automatically filling tables in like that, you're talking "programming." In this case, I would use a RecordSet to populate the positions subtable.

To use an analogy, if we were talking about naval vessels and officers, you might want to have a captain, commander, engineer, and medical officer on each ship, your tables might be:

ShipT: ShipID, ShipName
PositionT: PositionID, PositionName, Required
OfficerT: OfficerID, FirstName, LastName

So your ships might be:
1, Enterprise
2, Voyager
3, Defiant

Your positions would be:
1, Captain, Yes
2, Commander, Yes
3, Lieutenant, No
4, Engineer, Yes
5, Doctor, Yes

Your Officers could be:
1, James Kirk
2, Mr. Spock
3, Bones McCoy
4, Hikaru Sulu
5, Pavel Checkov
6, Montgomery Scott
7, Kathryn Janeway

I would use the "Required" field in the Position table to indicate if this position is required, and if so, it will be auto-populated when we create a new ship. You could alternately have a "NumberRequired" field too, and add more than one... but I digress.

Now, you need a cross-reference table for each Ship and the Officers. You can pre-populate this table when a new ship is created using a Recordset.

ShipXOfficerT: ShipID, PositionID, OfficerID

When you add the new Ship into your ShipT table, the recordset could populate the subtable by looping through all of the records in the RankT and adding them to the ShipXOfficerT:

1, 1, _
1, 2, _
1, 4, _
1, 5, _

Now you have four blank records (one for each REQUIRED position) and you could show those in a subtable with combo boxes, and now allow the user to select officers to fill in those blanks.

1, 1, 1
1, 2, 2
1, 4, 6
1, 5, 3

See how easy? The hard part is populating the table with the recordset data, which I cover in my Access 320-329 series. In fact, this is almost exactly like the lesson I give on student attendance in Access 325.

Hope this helps. Boy I love it when I can use a Star Trek analogy to answer a question. :)