There are three types of powerbanks which are available in the market
Those which advertise their capacity in mAh for the max voltage attained to the lowest voltage usable range. In short providing the capacity YOU can use.
The second type advertise their capacity in mAh from max attained voltage to 0V whole of which you can’t use or you may apply some hacks and use it and get done with the powerbank forever or come close to doing so( destroying it beyond use by using it till 0V which could be revived, but still some reduction in capacity will be seen if you do so) .In short providing more than the capacity you will be able to use.
Then there are powerbanks providing outrageous values for capacities like 50, 000 mAh 100, 000mAh or even better 30, 000mAh. But truth is these powerbanks are made mostly of 3.7V batteries which possess a max charge capacity of 3600mAh( Highest ever made) . Most powerbanks contain 3 to 6 of them and highest cpacity powerbanks are 20800mAh while there are a few 30000mAh genuine ones available. Above this it is crook with often 100, 000mAh powerbanks being just 10, 000mAh and 300, 000mAh’s being 3000mAh ( LOL) . If you call the companies they will tell you spelling mistake and you will have to know some foreign tongue to speak with them and once you learn the language they will tell you two words and you won’t have an answer.