An Observation Interval (OBI) is an uninterrupted observing interval for a single ObsId. An ObsId may be broken into more than one OBI for a variety of reasons, one of them being that the original observation did not get enough good time.

An in-depth discussion of multi-OBI observations and how to analyze them can be found in the Multi-OBI Observations why topic.

There are many ways to refer to an observation - Sequence Number, ObsID, etc. Here is a brief overview of how these pieces fits together:

   Proposal Number maps to
        N x Sequence Numbers maps to
	     M x ObsIDs maps to
	          Q x OBIs

where it is almost always true that N == M (i.e. there is one ObsID per Sequence Number).