How to use coverage amount in Usage contract line in OKS

I´ve noticed that in the log of the Service Contracts Main Billing program, there’s a parameter named Coverage Amount.

For instance, when billing January, the log shows as it follows:

date from 01-JAN-13To  31-JAN-13

Coverage amount    : 0

 Coverage start_date: 01-JAN-13

 Coverage end_date  : 31-JAN-13

counter 600

l_qty

l_init 300

After counter values X_value 300

Cll IB Lock 24-JAN-13, Counter id 293

Level   S

Price Calculated:  306

 

So far, all contracts I've billed, the coverage amount always shows as zero. I would like to know what is it for, and the way to set it for a specific Usage contract line.