The Open Measurement Interface Definition (OMID) uses the ad session as its unit of measurement: the integration creates a session and declares the creative type and impression criteria, and the OM Service dispatches session events and geometry data to verification scripts for the lifetime of that session. This chapter expands into three sub-pages: lifecycle & ownership, access modes & geometry, and impression criteria.
Session states and ownership from creation to finish: AdSessionType (native / html / javascript), the split between SessionOwner and EventOwner, the call order of start / impressionOccurred / creativeLoaded / error / finish, and the degradation decision made by isSupported().
The AccessMode determines whether a verification script receives a reference to the creative element: in full mode the OM Service supplies slotElement / videoElement in the context, while limited mode supplies geometry data only; domain mode was removed in 1.5.4 and falls back to limited when selected. The geometryChange event carries adView / viewport / obstructions, and the Reason enum explains why a measurement is not possible.
The ImpressionType declares under which condition a session counts an impression: definedByJavaScript / unspecified / loaded / beginToRender / onePixel / viewable / audible / other. Together with the CreativeType it determines the measurement methodology available to verification scripts. Declaring the criteria exists so that impression discrepancies between measurers can be explained.