Does Esri reserve short names for an ArcGIS Online instance?

Answer

No, Esri does not reserve short names for an ArcGIS Online instance. Most organizations prefer to use the acronym of their organization as the short name. However, this may not be an ideal choice for short names because it is possible for two different organizations to have the same acronym. Short names are provided on a first-come, first-served basis. Any individual or organization with a subscription, a trial account or a personal use account can choose to use any short name as long as it is available. If the organization or individual account subscription expires, the short name is returned to the pool and becomes available after 30 days.