When you join you may automatically be given Promotion membership. This grants the same accesses and privileges as Premium membership. It allows you to grab a sneak peak at what it is like to be a Premium member on this site.
At the end of the promotional period, your account will be transferred
to Standard membership or
If have in the mean time purchased premium membership, your account will be transferred to Premium membership.
In either case,you cannot renew Promotional membership. You should only be aware that your accesses will change when your Promotional membership expires, unless you have purchased Premium membership.