BIP 325 separates transaction validation from permission to produce blocks on a particular Signet. The default network uses signatures; a custom condition may have a different solution.
A Full Node checks Proof of Work, transactions and the additional Signet condition. A valid header alone does not prove satisfaction of the condition carried in the block body. [BIP 325 — Signet] [Bitcoin Core — Signet]
The condition is a script and may require one signature or Multisig. A custom Signet with OP_TRUE permits an empty solution; saying every Signet always requires a signature is inaccurate. [BIP 325 — Signet]
BIP 325 places a nonempty solution in the optional data of the BIP 141 coinbase commitment. The marker 0xecc7daa2 identifies the data; this is not a separate ordinary payment to the signer. [BIP 325 — Signet]
Verification uses two virtual transactions and a modified transaction root with the solution emptied. The signature covers neither itself nor the header nonce; searching for Proof of Work need not repeat signing for every nonce change. [BIP 325 — Signet]
Bitcoin Core provides a default Signet, but participants can agree on their own script. The message-start identifier derives from the condition; the name Signet alone does not identify the same network. [BIP 325 — Signet] [Bitcoin Core — Signet] [Bitcoin Wiki — Signet]
The goal is not guaranteed uninterrupted service. Coordination allows outages and reorganizations to be tested under more predictable conditions than uncontrolled test mining. [BIP 325 — Signet] [Bitcoin Core 0.21 release notes]
Signers can withhold blocks or restrict transaction inclusion. Their approval does not replace the node's other validation rules; this model must not be presented as mainnet security. [BIP 325 — Signet] [Bitcoin Optech — Signet]
Regtest can also connect multiple machines, but block production is cheap and easily controlled by a participant. Signet also targets long-running shared tests with defined block-production authority. [BIP 325 — Signet]
Signet coins are not mainnet BTC and the protocol does not redeem them for it. Addresses can share a test format with another network; looking at an address alone cannot establish the correct network. [Bitcoin Core — Signet]
Check network configuration, signet_challenge in getblockchaininfo and actual transaction confirmations. A successful wallet test does not prove resistance to an economic attacker on mainnet. [Bitcoin Core — getblockchaininfo RPC] [Bitcoin Core — signet miner]
For the clearest picture, read this entry together with Nakamoto consensus, Block propagation, Initial block download, Block weight. The reverse links also lead from Testnet.