They are arbitrary but they are also between ranges of numbers depending on the kind type. From NIP-01: ### kinds ... And also a convention for kind ranges that allow for easier experimentation and flexibility of relay implementation: - for kind `n` such that `1000 <= n < 10000 || 4 <= n < 45 || n == 1 || n == 2`, events are **regular**, which means they're all expected to be stored by relays. - for kind `n` such that `10000 <= n < 20000 || n == 0 || n == 3`, events are **replaceable**, which means that, for each combination of `pubkey` and `kind`, only the latest event MUST be stored by relays, older versions MAY be discarded. - for kind `n` such that `20000 <= n < 30000`, events are **ephemeral**, which means they are not expected to be stored by relays. - for kind `n` such that `30000 <= n < 40000`, events are **addressable** by their `kind`, `pubkey` and `d` tag value -- which means that, for each combination of `kind`, `pubkey` and the `d` tag value, only the latest event MUST be stored by relays, older versions MAY be discarded. ...