Single channel per carrier (SCPC) refers to using a single signal at a given frequency and bandwidth. Most often, this is used on broadcast satellites to indicate that radio stations are not multiplexed as subcarriers onto a single video carrier, but instead independently share a transponder. It may also be used on other communications satellites, or occasionally on non-satellite transmissions.
In an SCPC system, satellite bandwidth is dedicated to a single source. This makes sense if it is being used for something like satellite radio, which broadcasts continuously. Another very common application is voice, where a small amount of fixed bandwidth is required. However, it does not make sense for burst transmissions like satellite internet access ortelemetry, since a customer would have to pay for the satellite bandwidth even when they were not using it.
Where multiple access is concerned, SCPC is essentially FDMA. Some applications use SCPC instead of TDMA, because they require guaranteed, unrestricted bandwidth. As satellite TDMA technology improves however, the applications for SCPC are becoming more limited.