FrSky offers two types of sensors.
The X series have Smart Port and work with the X series receivers, no hub is needed. Just plug in the sensor and you are ready to use it, want to add another sensor, just plug it into the last sensor. Sensors can be in any order.
The D series sensors work with the older D series receivers and may require the use of a hub. These sensors are largely discontinued at this point.
There are a few sensors that work with both series. The basic voltage sensor will work with most all of the X series receivers (not the X8R). The varios can be used with either series.
FrSky's newest series of sensors are a hub-less design. No more hubs, the Smart Port allows you to daisy-chain multiple sensors together to meet your needs. This results in less clutter and gives the freedom to design the system you need, not what a hub will allow.
Requires a Smart Port equipped receiver such as the X series from FrSky. (X8R, X6R, X4R, etc.)