ObsID: Observation Id
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.
There are many ways to refer to an observation:
where it is almost always true that N == M (i.e. there is one ObsID per Sequence Number).