The process client uses an iframe to show the activity page. It uses an in-page anchor (#) to keep track of this. The anchor is the URL of the iframe, encoded using URI encoding and base64.
Using an anchor allows the pages to be cut and pasted as you would expect.
Session cookies are also used to remember the anchor, so that if the user session times out they are taken back to where they were.
However, using an anchor means that if you need to sign in then the sign in process (which captures and redirects you to the page) loses the bit after the #. This is a fundamental restriction - the hash value is not sent to the server so the server cannot capture it.
To allow for this, the process client allows you to set an a parameter (a for activity). If it finds this, it will redirect to itself having set the anchor. This is used for external links to the activities, for example in emails.