When false (default), getHeadBlockNumber will load the most recent block number. When true, getHeadBlockNumber will return the block number of the most recent irreversible block. Keep in mind that getHeadBlockNumber is an abstract method and this functionality is the responsibility of the implementing class.

For positive values, this sets the first block that this will start at. For negative values, this will start at (most recent block + startAtBlock), effectively tailing the chain. Be careful when using this feature, as this will make your starting block dynamic.