Sounds to me that somebody just would like to get everything ready for him…
There are lots of materials in wiki:
You should start with:
http://wiki.mikrotik.com/wiki/Packet_Flow
Then you should be clear on what happens where
next thing to read and understand should be:
http://wiki.mikrotik.com/wiki/HTB
It will answer all your questions about priorities and limits.
Then to make sure nothing is missing read:
http://wiki.mikrotik.com/wiki/Queue
http://wiki.mikrotik.com/wiki/PCQ
http://wiki.mikrotik.com/wiki/Burst