Some time after asking this question, I found the answer by chance (as I had stopped searching), I saw a guy talking about some essential features, among the features was there! MARKPLACES < 3 ...
It was love at first click, and I discovered a thorough Google documentation!
Some applications are required to display large numbers of locations or markers. Naively plotting thousands of markers on a map can quickly lead to a degraded user experience. Too many markers on the map cause both visual overload and sluggish interaction with the map. To overcome this poor performance, the information displayed on the map needs to be simplified.
Everything I wanted to know about the grouping of marks is there, very good and I recommend it!
Making a WordPress plug-in with this documentation made it easy!