Use 'tack onto' to describe something added surprisingly or as a last-minute thought.

Unexpected Additions

'Tack onto' implies the added part was not originally considered integral to the item.

Non-Integral Parts

'Tack onto' can seem casual and might be less suitable in strictly formal contexts.

Something 'tacked onto' often does not seamlessly integrate with the original content.

Using 'tack onto' can imply that the addition was hasty or inappropriate.

'Tack onto' suggests the add-on may not contribute effectively to the main subject.

When something is 'tacked onto', it may seem like an afterthought or less important.