Boosts are items that are dropped directly from monsters, looted from boss chests, or bought in shops; they permanently increase the stats of the current Shell according to the type.
There are currently 5 different mark levels of boosts, indicated with a small roman numeral, with each dropping from certain zones. Each Shell has a differing amount of boosts of each mark level that can be collected. The player's current Shell starts at Mark I and upgrades to the next mark level when all slots for boosts at the current mark are filled - when this occurs, a special effect occurs which deals (new mark level * 1000) damage to all enemies in a large area around the Shell. The maximum mark level is Mark VI, achieved by filling in all Mark V boost slots on one's current Shell.
When a boost of a given mark level is collected, it fills a slot in the lowest mark level of that stat that still has empty slots as long as that mark level is not greater than that of the boost. For example, if one were to collect a Mark II Base Crit Chance Boost, but had yet to fully fill in Mark I Crit Chance, that boost would take up a Mark I slot instead of a Mark II. However, collecting that same boost when all Mark II Crit Chance slots have already been filled does not grant a stat boost, as Mark II boosts cannot fill Mark III or higher slots.
The mark level of boosts that drop is based on the difficulty of the easiest version of the level- for example, medium levels Fungus Cave and Vanquished drop Mark II boosts. However, medium level Elite Fire Forest drops Mark I boosts because the normal Fire Forest is an easy level. Note that elite levels drop boosts at a much higher rate than do non-elite levels to make up for the increased difficulty.
Below is a list of what boosts can be found in each mission in StarBreak. Note that chests in secret areas give all boost types of a given mark level, not just the ones that are found in the mission in which the secret resides (e.g. Underground Base secret area chest gives all Mark III boosts, including those that are normally found in Strands).