If you research "add all option to combobox" you'll get lots of leads, many of which involvel lengthy functions. The premise of dependent combo or listboxes is that child controls are not populated until a selection has been made in the parent. OK, then how do you get the child box to display all the records if the first one is always going to be a filter for the second? The answer is to add an "all" option to the first.
A simple way would be to have "all concepts" as the first record in your table name, provided that it will always be first alphabetically. If not, you can ensure it will be by adding a numeric sort field to your table, but if there are a lot of records, or the list will change frequently, this would not be a good method. I increment sort fields by 5 so that I have room to insert something later without having to re-define the entire list for one new entry. The way to use this method follows the next method.
Here's a short method I found that doesn't involve table mods which I adapted to your situation but have never tested. I assume your first UnitIDKey records are text data type. In design view, make the following the source for UnitIDKey and it should add "all concepts" to the control records. Again, this value has to be first, alphabetically speaking. You can drop the sql into the property sheet rowsource for UnitIDKey or make this a stored query and add the query name.
SELECT TBLConcept.ConceptID, TBLConcept.ConceptCode & " - " & TBLConcept.ConceptDescription AS ConceptCodeDescription
FROM TBLConcept WHERE (((TBLConcept.UnitIDKey)=[Forms]![FRMTests].[UnitIDKey]))
UNION
SELECT Null, '(all concepts)' FROM TBLConcept ORDER BY ConceptID; NOTE: not sure about sort; you don't show one in your example
ConceptIDKey has no rowsource if you look at it in design view. Using the first combobox AFTER UPDATE event, you set the second control source progammatically. Use a Select Case structure or
Code:
If Me.UnitIDKey = "all concepts" then
Me.ConceptIDKey.rowsource = "a sql statement here"
else
Me.ConceptIDKey.rowsource = "the other statement here"
Me.ConceptIDKey.requery
end if
You should always back up your db or affected forms/tables etc first before trying design changes.