This category holds all research that boosts the strength of units already available to build. It is important to note that stat boosts do not have a compounding effect. For example, each of the 5 land training upgrades increases the health and damage of land units by 10%. So, if all 5 are researched, it is equivalent to a single 50% upgrade for both health and damage, instead of the 61% upgrade that would result from compounding.
Training upgrades are the only area of the research tree that affect experimentals.