limited entries in f4 help in selection screen

hi experts,

i have a selection screen and it has f4 help also .there it displayes all the records in the table .but i need to display only selected records.i have do it from the code itself.

for ex .if i have a selection screen to select matnr from mara table .i need to display the materials which fall under the category raw material alone .for that what i have to do ?