Can you reference another cell to be used as part of a custom number format? For instance, the spreadsheet will be used repeatedly for several projects, and the format of the the cell value will depend on the project. For example, the projects are A01, A02, etc., and for the purposes of a change log, the changes are numbered C-X01-001 for A01 or C-X02-017 for A02. (Note: the A01, A02, etc. are already custom formatted as A0#, where only the number is input.)
With your permission, if I can recommend. It can help us all if you upload an Excel file (without sensitive data), no picture. Even if it is said that a picture can say a thousand words, it is certainly not in the case of Excel, on the contrary in some cases. This would also be a blessing for all of us, as we can understand the problem much better, a win-win situation for everyone.
* Knowing the Excel version and operating system would also be an advantage.