The guideline as I understand it is that invoice numbers should come from one or more sequential series (ref) - nothing wrong with splitting them by month/year/type/whatever, the point is that whatever numbering you choose needs to be such that if your affairs are inspected then they can tell you’ve given them the complete set of records and can spot anything that’s missing. For example if they see 2018-AB-0757 and 2018-AB-0759 but not 2018-AB-0758 then that will raise eyebrows.