Use 'tag on' to indicate optional additions that are not critical to the main item.

Non-Essential Addition

'Tag on' often implies something was added later, not part of the original plan.

Afterthoughts

It's used to casually add extra information in a conversation.

Suggests someone is joining in without a clear invitation, informally or out of convenience.

Implies a social or physical act of following someone to be part of their plans.

Can indicate a lack of active involvement; someone is present but perhaps not engaged.