Hi Harsh
Main difference between F and B are
if the vendor is same for po and freight po then we can select the F category
if the courier vendor is different then the Purchase order vendor then we need to select the B delivery cos.
simple: supplier and shipping by same vendor F freight
supplier by one vendor and shipping done through another vendor then we need to select B delivery cost , for this category we need to create separate vendor.
I hope you got the answer to your question
Regards
Sudhakar