p.s so basically bub being 5/5 above your brim means it isn't engaged yet, meaning it's head isn't in your pelvis. 4/5 means 4/5th of bubs head is outside the pelvis, and 1/5 of its head is in your pelvis =) bubs getting into position =)
That's what my OB explained to me when I asked him that exact question last week!

Also, wrt position of baby - my understanding from birth classes is that it works best if the baby shifts around as it moves through the pelvis - for the top opening to the pelvis, it works best if the baby is facing the left or right side. As the baby moves further down (through the second part), it works best if the baby is facing forwards/backwards. This is due to the different parts of the pelvis having different dimensions (ellipses but with the long part facing different ways), and matching those dimensions to the babe's head shape.

But when it comes to birth, it's best if the baby faces backwards - and most, but not all, do. If the baby is facing backwards at birth, you can end up with "back labour" (ie. more back pain) and can be slower and more difficult.