You can use any tag you wish. However, the best practice is to think of tags as categories. Categories could be things like ‘COD’, ‘Corporate’, ‘Converted’ or ‘Lost’.
A reference number is a unique identifier, not really a category in its own right. In the case of a reference number, it may be more useful to add the reference number to the survey name field (where the customer’s name is usually entered, so for example: “#123456- Jane Doe”). It can then be searched for using the “Search Text” field shown below.