The #CONDITIONAL tag allows you to test a certain range of true/false conditions and specify different results based on the result. It takes the form:
<#CONDITIONAL type="[condition type]" test="[value to test"] true="[string to return if condition is true]" false="[string to return if condition is false]">
The true/false attributes are optional and will be replaced by an empty string if they are not specified.
You can use the <#CONDITIONAL> tag with the type attribute NotEmpty, and it will evaluate whether the value provided in the test attribute is a blank string or not. This is useful when displaying content is dependent on whether or not a field on an item, course, or user is blank.
This would only display the message about providing an email address if the user's email address is blank.
<#CONDITIONAL Type="NotEmpty" Test="<#USER field='EmailAddress'>" False="<p>You have not provided an email address for your account. Email is the primary means of communication with Reserves staff.</p>">