Field VBRK-MWSBK in VF01

Hi Gurus

Can anybody tell me how the field taxes (VBRK-MWSBK) in transaction VF01 is filled?

I imagine that it is composed through some conditions in the pricing, but I couldn't prove this theory to my user. He has a development (Z program) that reads this field and it is very important to know where it comes from.

Best Regards,

Adilson Ferreira