FreeHeadersRemaining: StorageDescriptor<[], number, true>

Number of free header submissions that we may yet accept in the current block.

If the FreeHeadersRemaining hits zero, all following mandatory headers in the current block are accepted with fee (Pays::Yes is returned).

The FreeHeadersRemaining is an ephemeral value that is set to MaxFreeHeadersPerBlock at each block initialization and is killed on block finalization. So it never ends up in the storage trie.