All that matters is relative size, relative placement at or near the center, placement on the field, and arrangement. There are two complications, though. In current SCA practice, an overall charge is never in the primary charge group. In the example in the definition, the fess is not the primary charge, even though it's a central ordinary. The lion is the primary charge.
Maintained charges are usually too small to count towards difference.
A maintained charge is never in the primary group. The maintained sword is by definition small and insignificant in SCA heraldry, so even though it's more central, it's not in the primary charge group. It's not always clear what the primary group is. Sometimes, it's simply a judgment call based on visual weight. Uncertainty about the primary can cause multiple possible and legal blazons, and it can affect the conflict checking.