Its all on the page describing the way double based elements work.... The notional rear of the front element is the one used for flee moves etc.... The other interesting thing to note is that compulsory double based elements per the lists are both killed where appropriate, voluntarily double based elements "make change" if one is destroyed. Also double based cavalry and lights do not pay per element for moves to the rear etc.
Actually, no. If you read page 13 you will see that the rear of the front element is only used for recoils, pursuits, and the zone of death when destroyed.
Otherwise it moves as if a single element, which means the entire base is treated as a single element, i.e. the front of the front is the front, the rear of the rear is the rear and the combined flanks of front and rear form the whole flank.
The line "It makes outcome moves as if a single element of the front type" is a bit confusing, but the context implies that this is a statement about what outcome move to make, not about how far to move or where to measure from, which the other bullet points cover.
The basic principle is that a compulsory double base is a single element, except for slight differences to make recoils etc consistent with single base elements and account for the fact it may contain different troop types.