Ann: Software Subscription TCV and ARR Update, page-22

I don't think i ever said it's ad hoc, per se, but to my understanding there is some ability of the customer to defer it, and it is influenced by when the software provider is releasing updates / when customers are asking for reconfigurations etc., so it can move around a little bit. I've also seen instances of other software providers stipulating a minimum annual maintenance/support spend (not sure if applicable to RPM) - it may well be that they do have a minimum in there given how stable it is. In a pinch you do get customers asking for deferrals etc. on support/maintenance - you can see that RPM's maintenance/support was flat from 1H15 to 2H16 during the trough of the mining downturn, for example.

I view 'subscription' as slightly higher quality than 'maintenance/support' for the reasons mentioned, but for shorthand looking at the sum of (maintenance + subscription) revenue would be the best way to gauge the business' underlying growth.