The USER tag is used by both the Borrowing and Lending web interfaces to display information about the user/library logged in. For Borrowing web pages the usable fields would be any field in the Users(ALL) table.
For Lending web pages the usable fields would be any field in the LenderAddresses(ALL) table. Note that while the field is in LenderAddresses(ALL), the tag would still be a USER tag.
Common places to use these fields would be on any form where you wanted to remind a user what value they entered upon registration.
For example, you could remind users that if you need to contact them about any request you will email them at:
<#USER field="EMailAddress">