Category code in PR

Hi ,

User creating the purchase requistion in AX 2012 , filled the header information and in the line level selected the item code and qty .

But it is throwing the error category code must be filled in .

a) Can i know where to fill the category code ?

b) when i select the vendor code in the PR line it is not showing any vendors ( even though database has 55 vendors ) What would be the problem ?