Yes, I think that officials at the Department of National Defence would be in a better position to explain the complex workings of an envelope.
As a complement to that, I worked as a public servant in the Department of Finance and in the Privy Council Office, and it was always challenging to figure out how much was left in the capital envelope at the Department of National Defence because of the multiple projects that are funded by these envelopes and the regular or frequent re-profiles that complex procurement projects encounter.
For these reasons, I think the Department of National Defence would be in a better position to answer your question as to how exactly it works.