I am developing a tracker for the POs raised by our Project Management department.  Each PO falls into one of 3 categories: Projects (P), Asset (A) or Overhead (K).  
Each category has a number which must be selected to indicate _which_ project, or _which_ overhead account, etc, the expenditure goes to.  The number of options at this point range from about 10 to almost 200, depending on the category.
Is there a way to set this up so that once the category is chosen in one Combobox, the options available in a second 'PAK' ComboBox are limited to the options in that category?  
Currently the 'PAK list' is in three separate tables, one for each category, but can be combined if necessary.