Reach and Frequency, for those who are interested listeners and radio aficionados, is the determination of how to use an ad budget to reach the highest number of target consumers the desired number of times.
Reach is desirable because advertisers want the campaign to reach lots of different potential consumers. Frequency is a goal because advertisers know that repeating the message creates awareness and thus sales. A criteria here is "share of voice" where an advertiser needs to know the frequency and percentage of ads in a category its competitors have.
In a reach and frequency example, a campaign may wish to reach 50% of the total target consumer group at least three times during each week of the campaign.
Stations with higher TSL require fewer spots to get to the same r&f goals. Two stations in the same format may duplicate cume so much that buying both may increase frequency, but contribute little to reach.
Station rates figure in, as it's important to reach as many people at the desired frequency at the lowest cost. So good stations may be skipped, as they might have the right delivery, but at too high a cost.