Visa confusion

I am thinking of travelling to Tibet at the beginning of July. My residency permit expires at the end of June and I am going to extend it into a L-visa. I have been told that I cannot do this until the last couple of days of my residency permit (although please do correct me if I may be able to do this earlier!)

Is the Tibet Travel Permit linked to a particular visa or residency permit - can I apply for it even if I don't yet have the visa I will be using? I'm not sure I will have my tourist visa 20 days before I am planning to travel to Tibet...