FlexPDE version numbers are of the form 6.00, representing the major version and minor version numbers.
When the revision is significant (such as adding moving mesh capability) the major version number (the first digit) is incremented.
When added features are an evolution of the existing version, such as adding new meshing capabilities, the minor version number (the second digit group) is incremented.
(Note: Occasionally versions appear in our download directory with an additional alphabetic subscript, like 6.01a, or versions with an "x" subscript, like 6.01x. These are experimental or special purpose sub-versions, and cannot be trusted. They are never linked through the main download page.)