Hi!

The periods you define in Planning Period Units window are used by Period Templates, which you define and maintain in Period Template Maintenance window. And Period Templates are used by Master Scheduling (primary usage),  MRP and Customer Schedules, to group data into different time buckets. It is not a MUST, but there are some graphs in Master Scheduling that requires that you have relevant data in Planning Period Units and Period Templates.

Two examples of Period Templates:
1) I would like to group my master scheduling data in different analysis windows by weeks, with a twelve week horizon.
    Then I can create a Period Template with 12 periods, one period each, using the Plan Period Unit: Week

2) And sometimes I would like to group my master scheduling data in different analysis pages in the following more advanced way; First 4 weeks, then 3 months one period each, and finally 3 quarters, one period each
    Then I can create a Period Template with:
  4 periods (one period each) using the Plan Period Unit: Week
  3 periods (one period each) using the Plan Period Unit: Month
  3 periods (one period each) using the Plan Period Unit: Quarter

From Apps10 it has become more easy to maintain you Planning Period Units data. Please try out the RMB menu here. I really suggest you to get rid of data that are older than let say 2-3 years (unless you have some special customization in this area). Having lot of old data in this window just slows down searches etc etc.

Hope this helps!

-Mats