The Lockdown is a stable, static release recommended for new users, or people trialing the software. The Development version contains developing features and is released quite often so people can obtain new (but untested) features and capabilities.
Lockdown: Mach3 R3.042.020
Release Candidate: Mach3 R3.042.021
Development: Mach3 R3.043.000 - This Rev is for Development testing only!
RC