Hi,
if you hover your mouse pointer over the name of a quantity in the Block Details pane, the description of that quantity will pop-up.
There's another way to make this "description" field permanent, in the same pane - please take a look at the User Guide.
The easiest way to understand what each model does is to select the block on the canvas and use the F1 key: this will bring up the corresponding page of the Models Guide.
In any case, I cannot tell you the meaning of the timer's variables in relation to another model (PI controller in your case), because all depends on how the two block are connected.
However, u1 is the 1st output signal, and subscripts 1, 2 correspond to the phases, i.e. the value of u1 in phase 1 and in phase 2 respectively; T1 and T2 indicate the two phases.
