The LL P2K engines with decoders installed are compatible with any NMRA compliant DCC system. This includes (but is not limited to) Digitrax, MRC, Bachmann, CVP, Atlas, Lenz and NCE. There are probably others out there that I am forgetting, but those are the most common ones I have knowledge of. Basically, the better the DCC system, the better it will be at handling the advanced features of certain decoders. This is especially true with sound equipped locomotives, since not all DCC systems offer control of all available functions that control sound.
For example, the Bachmann EZ Command system does not offer much in the way of functionality or programming ability, but it is by far the least expensive system available at this time. At the other end of the spectrum are the NCE Procab systems (if I recall correctly) and the Digitrax Super Empire Builder systems. These offer just about every bell and whistle (pun intended) that has been thought of to include in a DCC system.
I know I've added a lot more info than just the answer to your question, so I hope it has been useful. Speak up again if you need more information, especially if it's specific to a particular DCC system that you may have in mind.
-Joe