In Ripple, what is "Total XRP" in the ledger header? In the spec for the ledger format there's a field called "Total XRP" in the header. Is this the total number of XRPs in circulation? Does this value get decremented with each transaction?

Can this value be incremented as well?
This is the total amount of XRP in the ledger. It is decremented with each transaction that pays a fee (which is all of them except for a few special transactions). There is no way it can be incremented.