Architecture
From CTWUG Wiki
The Architecture Name will give you an indication of which ROS version to download and then install.
Below is where you will find this info. Similarly, you could do this through telnet and get the details by just typing "system resource print"
The Architecture name can be seen in the picture below in the green circle. My router is a RB133 so as you can see the architecture name is mipsle so the file i would download would be, for example: routeros3.x-mipsle.npk
