From the Esri link here http://doc.arcgis.com/en/arcgis-online/reference/faq.htm
It says it is performance related
Premium feature data stores are available in three levels of performance: M1 (improved performance), M2 (higher performance), and M3 (highest performance).
And to phone ...
Contact Customer Service or your account manager for pricing, performance information, and availability.
But I presume that you would have a more direct line to AGOL support info
