Anything under 500 tons is classed as a fighter and can't be maintained by maintenance facilities - only by hangars.
This rule was to prevent a couple of maintenance facilities supporting a lot of fighters. In C# Aurora, fighters can be maintained by maintenance facilities as the tonnage supported is based on the total ship tonnage, not max hull size.
What a weird rule. I thought it was to prevent mineral consumption.
With consideration to that, actually, won't fighters incur a much more significant mineral drain on those who use them due to maintenance costs, now? As I remember hangars no longer being our legendary mothball for the change.