Random seed-You're not using speedrun mode,playing the mission through the regular choosing in the MISSIONS menu.There is no seed under your time.

Set seed-You're using speedrun mode,Speedrun mode allows you to choose a seed which determines for example: -Where specific things are.Like power box in the financier,safe in deposit.Well you get the idea. -Npc placement at the begggining,etc. Basically there is Seed:(insert number) Under the timer.