msp430f6659 - what is FLASH and FLASH2

Memory below 10000h can be accessed with a 16-bit address pointer and is as fastest. The other part needs a larger address pointer and is thus slower to access.
Some sections can only be accessed with a 16-bit pointer, such as Interrupt Vectors and must be placed below 10000h.
If you wish you can join the segments together to get one large contiguous section.

Most is probably already done in your current ‘Linker Command File’ but for more information download: SLAU131 “MSP430 Assembly Language Tools” and SLAU132 “MSP430 Optimizing C-C++ Compiler User's Guide”.