The stations I describe are digital sub-stations.
A broadcast transport, which I suppose corresponds spectrum-wise to an old analog station, has* something like 38 mbps of video bandwidth. An HD station uses about 19.2 mbps of bandwidth, an SD station around 3.75 mbps.
So, a broadcaster with a transmiter and a transport can not only show an HD station, but one or more SD stations.
In Portland, the NBC station has 8.1 (NBC programming in HD), 8.2 (mostly news and weather) and 8.3 (Estrella, Spanish language programming).
OPB public broadcasting has 10.1 (HD PBS programming), 10.2 (repeats of PBS, plus artsy crafty programs, local author interviews. old movies) and 10.3 (pretty pictures plus OPB radio).
The ABC station shows MeTV on 2.2, the CW station has Antenna TV on 32.2 and ThisTV on 32.3. A religious programmer has SEVEN SD substations, 24.1 to 24.whatever, all with horrible non-stop televangelists and creepy low-budget childrens' programming.
You can think of most of the substations to the broadcast equivalent of a cheap mostly-advertising magazine that your local newspaper occasionally folds into the daily edition. The newspaper gets a small fee; the magazine editor gets low-rent eyeballs.
*Assuming they are similar to cable QAM transports.