Rule 46(b) is one sentence long, and it is enough to reject an invoice at the IRP or a Table 13
A GST invoice number must be consecutive, no longer than 16 characters, made only of letters, numbers, hyphen and slash, and unique within the financial year.
Multiple series are allowed, so branches, product lines or channels can each have a prefix, as long as each series is consecutive and reported separately in Table 13.
The IRP adds its own checks: no number starting with 0, slash or hyphen, and no duplicates for the same GSTIN, document type and financial year.
Every document type (invoice, credit note, debit note, delivery challan, bill of supply, vouchers) carries its own series under the same rules.
Sixteen characters, counting every letter, digit, hyphen and slash. A prefix such as INV/27-28/ uses ten of them, leaving six for the serial.
The full guide has the formats, the deadlines and the worked examples.