Custom invoice numbering can be set at the global level and at the project level. The setting at the project level (Projects > General > Billing Options) takes precedence over the one at the global-level (Settings > Billing & Invoicing > Invoice Settings).
CORE supports both numeric and alphanumeric invoice numbers in a 'prefix - last number - suffix' format. Prefix and Suffix are static; only the number changes. You can see the preview of the numbering when this option is enabled. Remember, if you generate an invoice (or a draft) and delete it, CORE re-uses that number so that there is no gap in the sequence.