Random Lengths are just that random length boards that add up to whatever total linear foot you ordered. We do random orders in any size we select from 6′-22′ (although 6′ and 22′ would be very rare). If you need specific lengths for a project, you should buy specific length boards.
We, however, will try to get a good spread of sizes for the random length orders, but typically it is what we have currently overstocked. Let us know what your goals are, and depending on inventory we may be able to ship more or less of certain types of boards. However, on random length orders, we do not guarantee any specific amount of any particular length boards.