In communities platform, we offer 2 links to the members -
A portal link allows you to direct your members to your Community's home page where they can further Login/Signup based on their preferences.
The Recruitment URL allows you to direct your recruitment traffic to a URL where they have to first fill in the Profiling form and answer all profile related questions and then can proceed with further signing up.
This is solely done to filter the members per need.
The tracking variable is stored in a cookie. In general, you can use whatever symbols that browser cookies can handle. However, keep in mind that the link is in the URL - so if you have characters like "&" etc. they need to be HTML Escaped.
Our recommendation is to not put any special characters in the value of the source field. Values like "&" and "?" interfere with the HTML URL and may not be captured correctly.