You're not free to supply whatever letters you want - the words are specifically prescribed. The dew point indicator words are on page 41 of the handbook linked by the parent.

I'm not sure why the dewpoint indicators had to be arbitrary, rather than using the same scheme. Maybe it was a way of providing some robustness because fields could occupy variable numbers of words, so an identifiable dew-point word would help to mark a specific field in the report.