They don't make it easy to figure out but after researching it for my Co. this is what I came to.

    * One chat message -> one premium credit (most at 1 credit but some are less and some, like opus, are 10x)
    * Edit mode is the same as Ask/chat
    * One agent session (meaning you start a new agent chat) is one "request" so you can have multiple messages and they cost the credit cost of one chat message.

Microsoft's Copilot offerings are essentially a masterclass in cost opaqueness. Nothing in any offering is spelled out and they always seem to be just short of the expectation they are selling.