In some parts of this document Tn (where n can be 1, 2, 4, 8, etc.) is used to represent a clock. T1 indicates a root clock, while Tn clock indicates the T1 clock divided by n. For example, T1 divided by eight is called T8, its period is eight times T1, and its frequency is T1/8.
In most places, the T1 clock indicates the sampling clock, regardless of whether it comes from an external or on-chip PLL.