Aux Color is used when you enable Jitter, and set non-zero value in 'Aux Color' textfield in 'Jitter' section
P1 ... P4 - when transforming layer you usually modify it by dragging on canvas. But you can also set locations of the all corners by using a textfields - it can be used when high precision is needed
P1 - top-left
P2 - top-right
p3 - bottom-left
p4 - bottom-right
Pivot - pivot point used when rotating and scaling