Random number creation in asterisk

in the example given " exten => s,1,Set(junky=${RAND(1,8)}) " here the word junky what does it mean? with what do i have to substitute in my configuration?

If i have the following dialing extension "_[1-99]." saying that any number starting with 1 to 99 and followed by any number is called to Asterisk a randomly generated number should be created from 1-100. If the caller's number is small from 50 (from the generated number), let Asterisk answer and play a file, if the caller's number is bigger than 50 (let's say 86) Asterisk should drop the call. So my question is, how to generate random numbers in extensions.conf file and after generating the random numbers, how to use the generated number to be to accept or drop the calls?? Any kind of help I can get on this is greatly appreciated. Thank you in advance)))