Beginning Friday, May 25, 2018, Comcast will begin blacking out CBS Network programming on the out-of-market Designated Market Areas (DMA). This impacts the CBS affiliate, KYW (channels 3 and 1003) for the Lebanon, PA channel lineup.
KYW's signal originates in Philadelphia, PA in the Philadelphia DMA. Customers in the Lebanon, PA area (Lebanon County) are in the Harrisburg-Lancaster-Lebanon-York DMA (TV market). KYW is considered a distant signal and subject to blackout.
Comcast is required to blackout this programming at the request of the in-market DMA network affiliate, WHP (CBS Harrisburg). CBS network programming airing on KYW is also carried on WHP, although not necessarily at the same time.
The terms "DMA", "out of market", "significantly viewed", "network non-duplication" and "distant signal" all have extensive definitions and examples, some links below that will make for some good reading: