You should only need to set the child-parent relationship between the form and subform using the value in the combobox. For example:

Records:

A, B, C
B, B, B
C, B, A
B, A, B
A, A, A

If both the subform and the main form are based on the same table, and you want to display on the subform all records with A in column one (field one), then you set the parent and child fields for the forms to column one. If the main form contains nothing but a combobox where users select a value from the list, then if, say, C were selected, only the third record would appear in the subform. This shouldn't require any VBA.