Every Chandra observation is assigned an Observation Id (ObsID), which indicates that it is an observation of a specific target for a specific proposal with a specific observational configuration. All data for a given ObsId are always combined into a single set of Level 2 products. Each ObsId contains one or more Observation Intervals.

For more information, see the Multi-part Observations section of the Chandra Standard Data Distribution Contents page.

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).