Considering the amount of time it takes to make a Codex the wording on this upgrade is very misleading, the three levels say respectively that they:
"increase unit capacity by 5"
"increase unit capacity by 10"
"increase unit capacity by 15"
Well imagine my disappointment having spent weeks making a second Codex only to discover that, unlike every other upgrade that I can think of, they don't stack and it's actually increasing the unit capacity by 5 with each upgrade.
Wording should be either:
"increase total unit capacity by 5"
"increase total unit capacity by 10"
"increase total unit capacity by 15"
or just "increase unit capacity by 5" for all three levels of the upgrade.