BackStaff applications are capable of connecting to Historian software as a Historian Client.
A Historian is a stand-alone software or a module within a SCADA software that is capable of storing time series of values for thousands of variables, or tags as they are also known.
Historians store information in a relational database like MS SQL, Oracle, or MySQL. However, some Historians use a specialized file format, which optimizes the use of the hard drive space through compression. Anyway, every Historian can receive queries through SQL language.
The advantage of the SQL language is that you can ask for data based on very flexible filters that can correlate information not just from the same tag, but from others as well. For example, in an HVAC application, you can ask for all the temperature values and times when the temperature in the room was higher than 75 F, and the air conditioning system was on.
The idea behind this type of application is to permit the end users to do a tag versus time plot for one of many variables in a time span that can be from seconds to years.
Once you have your BackStaff connected to the Historian, you can do trends for every variable in the database. You can also create lists of variables you want to watch closely, we call such a list a scorecard.