MB1B another document is posted

Hi SAP experts ,

I am not an MM consultant but an ABAP so please bear with me.

When I do transfer postings in MB1B with movement type of 351, another document is created when I check in ME23N.

The document has a Z** movement type and movement type is labeled 'Post Goods Receipt for PO'.

I need help here as to why this document was posted.

Any input is highly appreciated.

Thanks and Regards,

Katherine Darunday