Relevant only if 'FastSync' is 'true'. If set to a value, then it will set a minimum height threshold limit up to which FullSync, if already on, will stay on when chain will be behind network. If this limit will be exceeded, it will switch back to FastSync. In normal usage we do not recommend setting this to less than 32 as this can cause issues with chain reorgs. Please note that last 2 blocks will always be processed in FullSync, so setting it to less than 2 will have no effect.