A User Activated Soft Fork (UASF) is a type of upgrade to a blockchain network that, if implemented in a sufficient number of “full nodes” (validator nodes; not miners) will not require the support of the majority. of the mining nodes.
This type of upgrade is part of the “Soft Fork” family and has the advantage of maintaining compatibility with non-updated nodes, unlike hard forks, which require all nodes that want to upgrade to be upgraded. continue to be part of the network to software compatible with the new consensus standards adopted.
The approach of U.A.S.F. as a means of updating the network is based on the theory that “full nodes” can act as representatives of the economic majority of the network and that an update implemented by the majority of them can force miners to adopt new consensus standards. because it puts them at risk of losing the reward of the mined blocks.
Usage examples for U.A.S.F. to add features to a blockchain network are the activation of the P2SH (Pay To Script Hash, better known as BIP16) function and the SegWit (Seggregated Witness) transaction format in the Bitcoin (BTC) network.
Soft forks are really very important within the different blockchains, since it allows users of a block chain to correct or improve the network without the need to create another cryptocurrency in the process.
Of course, a clear example is that of the DAI stable token that, thanks to a soft fork, can change its system from a single collateral in Ethereum to multiple collateral, allowing the creation of funds with other cryptocurrencies and tokens.
They are not really strictly mandatory, but if it is required better, the network is the only solution, since sometimes block chains require this type of solution to develop better.