Tibet and China

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Hi guys, Im planning my trip to China and Tibet. I know entering Tibet you need to get a permit but what exactly is the permit?

 

my question is, is this permit like a visa?

 

and if I leave China for Tibet do I need another visa to re-enter china or is Tibet part of China?

 

Apart from Tibet I wont be leaving and re-entering. So should I get a multi-entry visa or a single entry visa?

 

I know this doesn't have anything to do with Pattaya or Thailand but I'm hoping someone here can help

 

Thanks