Hi
Call horizon is used to propose a pre call date for the maintenance plan. for example if the maintenance plan is for every 1 month and you need to release the call say 3 days before ,
then you specify the call horizon as 90 % and hence it will release the call on 27th day. for releasing the call you have either run IP10 manually or IP30 regularly.
Say you started your maintenance plan in 1st of Nov 2013 and you required the call on 27th Nov , hence you can plan for resource and material between the 3 days, while the actual work you are taking is 30th November,. for this you schedule the IP30 regularly say every Sundays with the call interval of 7 days.
While the IP30 started on 24th November system will check all the maintenance plan based on the call horizon whether the call date is lies between 24/11/13 to 30/11/2013. if that is the case it will release the call
I hope you understood
Regards
Thyagarajan