Watt hours, milliamp hours, and what airlines actually measure
Airlines regulate watt hours, not milliamp hours. Wh is cell capacity times cell voltage, and the lines are 100 Wh and 160 Wh.
Two lines, drawn the same way by every airline.
mAh is not energy
A power bank sold as 20 000 mAh does not hold 20 000 mAh at the voltage your phone uses. That figure counts the cells at their own nominal voltage, typically 3.6 or 3.7 volts. Multiply the two and you get watt hours, which is the unit that describes the actual energy stored, and the unit every aviation authority uses.
The two thresholds
The thresholds are consistent worldwide: under 100 Wh you can carry it in the cabin without asking, between 100 and 160 Wh you need the airline's approval, and above 160 Wh it cannot fly as passenger baggage at all. Power banks must be in the cabin, never in checked luggage.
Where real banks land
A 20 000 mAh bank is usually around 72 Wh, comfortably fine. A 27 000 mAh bank is around 100 Wh and sits right on the line, which is why some are deliberately built at 99.9 Wh.
Wh = (mAh ÷ 1000) × cell voltage. Use the cell voltage, not 5 V, or you will overestimate by a third.
some airlines want to see the Wh figure printed on the casing. Not every maker prints it.