How differential backup works and impact of copy_only backup on Differential backup

Overview of differential backups

A differential backup captures the state of any extents (collections of eight physically contiguous pages) that have changed between when the differential base was created and when the differential backup is created.

The following illustration shows how a differential backup works. The figure shows 24 data extents, 6 of which have changed. The differential backup contains only these six data extents. The differential backup operation relies on a bitmap page that contains a bit for every extent. For each extent updated since the base, the bit is set to 1 in the bitmap.

Note 

The differential bitmap is not updated by a copy-only backup. Therefore, a copy-only backup does not affect subsequent differential backups.