"BAPI_PR_CHANGE" changes the item category of the PR Item

Hi,

We are using BAPI_PR_CHANGE in the custom program to update few fields which are custom fields. These custom fields are getting updated properly and also Item category is changing to Standard (0).

I would like to know why item category is changing to Standard (0) default, when we use the "BAPI_PR_CHANGE" to change the PR.

Regards

Hari