Planning Horizon for scheduling parameters for planned order

Hi, All PP Gurus........

1) can you tell me what impact does it make if i maintain planning hoirizon days in detailed schldng in OPU5.

What if i ll keep that field blank?

2) what are the prequisite setting for Capacity planning for work centers?

3)what is evaluation time profile for capacity?

Rgds,

Yogesh thorat.........