I have seen this namespace so many times but could not understand or I should say never tried to understand it.Today I have found this small piece of information from Deniles's blog.Just marked it here for my future reference.
"Sometimes you want to change the behavior of a Flex component but its hidden behind a private method or variable so you can't subclass it. Thankfully they provide the source so you can just create your own version. This is heavy handed and in some cases you just need to tweak things a little. Many components within the Flex framework have methods and properties flagged as mx_internal"