TTB paper here means the permit issued by Tibet Tourism Bureau; this permit (also called paper) is the first paper required to travel in Tibet;

there are only two conditions under which you only required to have this TTB paper

1. your stop in Tibet A.R. is Lhasa city either by air or by train

2. your entry into Tibet is through Zhangmu town (Kodari inside Nepal), the border town between Nepal and China, and your travel route is Kodari/Zhangmu - Tingri - EBC - Lhatse - Sakya - Shigatse - Gyangtse - Lhasa. (if you enter Tibet from Nepal, but you want to cover Mt. Kailash, lake Manasarovar, you are required to have other 2 papers: Aliens' travel permit and Millitary permit)