more importantly there are about 100 mA specified for all I/O lines.
Add them all up and they must not exceed 100mA.
the pins are splitted in groups, what's the reason for that
Groups have their own limits when it comes to output current.
however there are also maximum ratings but they don't excel 24 mA
These are limits where damage will occur.
Keeping the current below 15mA will be safe.
that I hooked up some standard LEDs that, require about 20 mA
Don't this is not a 'normal' arduino it hasn't got the current capacity to drive much directly.