It’s a statement by a retiring auditor that there are (or are not) circumstances that have led to the auditor’s decision to retire from that office
It could, for example, be a breakdown in mutual trust between the board and the auditor or a divergence in opinion about the legality of some proposed action
I’m not aware of any difference in significance based on whether the public company is quoted or unquoted – the statement of circumstances / no circumstances is required in both situations so far as I am aware