Generated Missions (Secondary Missions)

Anyone know how EOC decides when to give secondary missions (Generated Missions)?

Is it purly random? Based on Piracy level/skill? If so higher or lower skills?

I'm asking becauce I remember getting a mission about Big Jack's Jewels when I first started playing but i've not recieved it again since.

I've been investigating and found resource files for approx 6 sub missions most of which I've never encountered.

After some searching of the forums, I found people asking for solutions to some of these secondary missions so they must be playable.