Jane,
My personal understanding of what the purpose of pseudonyms is to create an alias address for accounts.
For example my account (username bob) might get 3 email addresses, bob@domain.com / sales@domain.com / users@domain.com now instead of creating 3 different email accounts, you create 1 account and (in this case) 2 pseudonyms, or as many as you want.
So now you only have to check 1 account to get all the emaill addressed to each of the 3 different addresses.
Mitel has made it so that when you create an account, it automatically creates 2 pseudonym's using the inputed first and last names. Which is great if you use them, but personally I do not, so I just delete them.
But I have made several pseudonym's for accounts, such as webmaster, sales, enquiries, etc etc etc. I think you can make as many as you wish, I _think_?
Hope that helped!
Cyrus Bharda