what causing goods issue material sometimes go to network activity and WBS ?

Dear PS Expert,

i have a problem when i do goods issue material, when i check detail of report S_ALR_87013542 under cost element that contain actual cost from goods issue, sometimes i found that the object type are NWA (Network activity) and sometimes are WBS.

what cause the posting can go to NWA or WBS?, because i want to block the possibility of the cost go to NWA.

Regards,

Karya