Sidechaining a compressor with reverb to get a nice reverb tail

"Key input" or "key signal" simply refers to whatever audio signal the compressor is listening to. It is also referred to as the "sidechain".
 
The key can be the same audio that's being compressed (internal sidechain), or can come from another source (external sidechain). All compressors support internal sidechains, but not all support external sidechains. The Sonitus compressor is one that does support external sidechains.
 
Whether you use an internal or external sidechain to accentuate reverb tails depends on where the reverb is inserted. In either case, it's still the vocal itself that serves as the key signal. Because vocal reverbs are normally placed on a separate bus (so the reverb can be shared by more than one track), that'll require that the compressor that follows the reverb plugin be configured for an external sidechain. In other words, the compressor is monitoring the lead vocal track to determine how much compression to apply, rather than its own main input.
 
To set this up, you'll need to add an Aux Send to the vocal track and route it to the compressor's sidechain input. Depending on the compressor you're using, you may also have to configure it to use the external key input. This isn't necessary for the Sonitus, but for, say, FabFilter Pro-C you have to explicitly tell the compressor to use the external sidechain.