Asked to elaborate, the spokeswoman for Microsoft said, "all unscheduled downtime, from the minute it starts until it is resolved, is counted against our SLAs," and that, likewise, there isn't a minimum number of users that need to be affected for the outage to be recognized.
She also clarified that neither the SkyDrive cloud storage service nor the Outlook.com webmail application are included in these Office 365 uptime measurements.
Office 365 Pro Plus isn't contemplated in the calculation because it's a cloud-delivered and cloud-updated version of the Office productivity suite that's nonetheless installed on users' local devices.
The Office 365 service level agreement is for 99.9 percent uptime, and calls for Microsoft to compensate customers financially if that guarantee isn't met.
Juan Carlos Perez covers enterprise communication/collaboration suites, operating systems, browsers and general technology breaking news for The IDG News Service. Follow Juan on Twitter at @JuanCPerezIDG.