Posted on March 04, 2018 at 08:46Hello
Arm Compiler 5 (AC 5) is the legacy ARM compiler that has been shipping for many years. It will continue to be supported.
Arm Compiler 6 (AC 6) is LLVM based and as such some modifications might be needed to port from AC5.
See
http://www.keil.com/support/man/docs/uv4/uv4_dg_armclang_cmp.htm
At some point in time the examples in the packs will be made for just AC6.
AC6 supports the new ArmV8-M Cortex-M23 and Cortex-M33 - not AC5. So these examples are written for AC6.
Most versions of MDK currently ship with both AC5 and AC6. You can select either one: 
Keil and ST do work very closely on the Packs and STM32CubeMX.
Thanks
Bob Boys
California