Difference between Tasks in Keil RTX using RL ARM and Threads in CMSIS RTOS

What is Difference between Tasks in Keil RTX using RL ARM and Threads in CMSIS RTOS?
I have found that in previous version of MDK the KEIL RTX used tasks with a max number of 256 tasks possible but when coming to CMSIS rtos it uses threads .so whats the difference between those. I am working on CAN in Keil MCB 1700 board with lpc 1768 chip. I got stucked as CMSIS Rtos has no CAN driver support but RL ARM has CAN support.

Which is better to use RL ARM or CMSIS RTOS?? and 2 more basic doubts
1. Difference between CMSIS RTOS API & RTX
2. Difference between Task & thread ?

Which I should choose?

Thanks in advance

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