A unit number looks like a label. It behaves like a join key: the shared value that lets two separate systems agree they are talking about the same door. Most rental operators have never decided what that value is, so every system picked its own.
Nothing errors when the key breaks
Your management system holds Unit 4. Your listing feed sends #4. The plumber's invoice says Apt 4. The utility account is 004. A person reading any of those knows it is one door. Software does not. It sees four units, or it sees no match at all and skips the row.
None of that raises an alert. The listing goes live a second time under a slightly different address and splits your traffic between two pages. The work order attaches to a unit record with no tenant on it. The turnover board shows a suite that was finished last week. You meet these one at a time, months apart, and file each as a one-off rather than as the single problem it is.
Canada Post already picked the format
For anything that leaves your office, you do not need to invent a convention. Canada Post's addressing guidelines say to put a hyphen between the unit number and the street number, and to skip the # symbol entirely. So 12-455 King St W. Use that on leases, notices, invoices and listings, and your addresses will survive contact with every mail and delivery system downstream.
Inside your own systems, do the opposite of decorating it. Store the unit on its own field, in one form, and let the software assemble the full address when it needs one. Bare digits where the building uses digits: 4, not Unit 4, not 004. The exact string the building uses where it is not numeric: PH2, B, 4A. Never pad with leading zeros. That is the most common silent break in the whole chain, because a spreadsheet strips them and a database keeps them.
One owner, one place a unit is born
The format matters less than who enforces it. Pick the system where a new unit gets created, usually your property management software, and make every other system copy from it. Nothing else gets to mint a unit by someone typing into a free text box. Free text is where Unit 4 (rear) comes from, and that string will outlive everyone who remembers the rear entrance.
Two habits stop it drifting again. Never renumber a unit in place, because the history stays attached to the old value and quietly stops following the door; retire the old key and issue a new one instead. And normalize a new building once, at intake, before the first listing goes out. Fixing it afterwards means chasing one suite through six systems that no longer agree on its name.
The audit takes about five minutes
Export three lists: every unit in your management system, every unit in your listing feed, and every unit that appeared on a work order in the past year. Sort them and compare them as text rather than by eye. Count the entries that show up in one list and not the others.
That count is the size of your problem. In most portfolios it is small, ten or twenty rows, which is exactly why it never gets fixed. Those rows are where a listing quietly duplicates, an invoice fails to match a property, and a finished turnover gets counted twice. Unit numbering is boring infrastructure. It stays boring only while one person owns it.