The 'BOLT' in 'BOLT11' refers to [Basis of Lightning Technology](https://github.com/lightning/bolts/blob/master/00-introduction.md) . It's once again a set of protocol documents for off-chain Bitcoin transfer. Similar to NIPS for Nostr and BIPs for Bitcoin.

[BOLT #11](https://github.com/lightning/bolts/blob/master/11-payment-encoding.md) describes the protocols for invoicing Lightning Payments.

It describes how the different segments of a Lightning Invoice should be formed. see [here](https://www.bolt11.org/)

For example `lnbc15u1p3xnhl2pp5jptserfk3zk4qy42tlucycrfwxhydvlemu9pqr93tuzlv9cc7g3sdqsvfhkcap3xyhx7un8cqzpgxqzjcsp5f8c52y2stc300gl6s4xswtjpc37hrnnr3c9wvtgjfuvqmpm35evq9qyyssqy4lgd8tj637qcjp05rdpxxykjenthxftej7a2zzmwrmrl70fyj9hvj0rewhzj7jfyuwkwcg9g2jpwtk3wkjtwnkdks84hsnu8xps5vsq4gj5hs`

- `lnbc` in the invoice is the prefix for Bitcoin mainnet. `lntb` for Bitcoin testnet, `lntbs` for signet and `lnbcrt` for regtest.

- `15u` is the amount where 15 is 1500000 and `u` is the multiplier (u = micro 0.000001 BTC, m = milli 0.001 BTC, n = nano 0.000000001 and then p is pico)

- `1` is separator

- `p3xnhl2` is the timestamp in 35 bits big endian. Can be converted to unix timestamp for reading.

- `pp5jptserfk3zk4qy42tlucycrfwxhydvlemu9pqr93tuzlv9cc7g3s` is the payment hash beginning with tag `p`

- `dqsvfhkcap3xyhx7un8` is description where`d` in the beginning is an indicator of it. This should read as `bolt11.org` in UTF-8

Then there are details about invoice expiry, payment secret, feature bits, signature and checksum. All of these together forms the Lightning invoice and that's in the BOLT11 protocl specs. That's why it's named that way. We can also just call it Lightning invoice.

Similarly, a draft 'offers' proposal called `bolt12` which improves on `bolt11` with reusable QR codes, blinded paths, schnorr signatures etc are available. Read more on bolt12 [here](https://bolt12.org/)