UASF: User Activated Soft Fork. Developers add a mandatory rule set to change the node’ software, invalidating certain kinds of previously valid blocks after a flag day. This method requires no mining majority to support or activate a chain-split. The UASF proposal intends to make a 51% attack against the blockchain that has the majority of economic activity, and this attack is called a “Wipe Out”.
UAHF: User Activated Hard Fork. Developers add a mandatory rule set to change the node software. These changes make previously invalid blocks become valid after a flag day, which do not require a majority of hash power to be enforced. Nodes with the rule set changes will follow this chain irrespective of its hash rate. The UAHF proposal is a peaceful and voluntary departure of different community members who have different opinions or visions, and it is not intended to make an attack against other blockchain(s), even if the UAHF chain has the higher hash rate.
BIP148 node: a Bitcoin node that has implemented BIP148 consensus rule changes.
BIP148 chain: a blockchain that is valid according to the BIP148 consensus rule changes. BIP148 is a kind of UASF.
Original chain: The blockchain that uses the same consensus rules in use today. (May 26th, 2017)
Wipe Out: If the UASF chain is activated and if the UASF chain gains the majority hash rate, then the nodes following the original chain will reorganize and begin to follow the UASF chain. In such an event, a significant number of financial transaction records will disappear. This is a risk that UASF nodes impose on nodes intending to follow the original chain. In contrast, a UAHF does not threaten the nodes following a different rule set with this same risk.
Bit 1: The BIP9 version bit in a block header used to signal for SegWit activation.
Stagnation risk: A blockchain without mining support may suddenly stop being extended, because the economic incentive for miners is low. A minority fork like UASF is under serious risk of permanent stagnation.