MPLAB® Harmony is the next generation of Microchip software libraries. These new libraries have been designed to be modular and interoperable and can easily work with an RTOS.
The older MLA is similar to Harmony in terms of the applications it supports (USB, TCP/IP, etc…) but these libraries were not designed for interoperability. Integrating MLA libraries requires more software development time to remove conflicts (peripheral access, function, resource, and variable namespace) between these libraries.
Microchip Technology acquires Atmel