Defining system scope and boundary
A system scope / boundary diagram is sometimes also called a level zero data flow diagram or a context diagram See here for the Wiki on DFDs and here for System Context Diagrams.

Whereas an ERD is a model of data at rest, DFDs are a model of data "moving" through processes.  Before RDBMS was the predominant data storage mechanism, data was kept in files that were accessed sequentially and was processed a record at a time.  Each time the data was processed it was written into a new file.  In this way, you could think of data "flowing" from one place to another, transforming as it went.  Each data store was shown - often as a rectangle.  Processes were shown as circles ("bubbles") and flows were shown as lines or arrows.

The level zero DFD shows the whole system as a single bubble.  Outside of the bubble are external actors (e.g. users of various types) other systems, and any other data sources and sinks that you might be concerned with.  

The idea of a DFD is to incrementally decompose your system into smaller and smaller processes, until the whole system is described.  The level 0/context diagram is the starting point and you drill into each bubble as deeply as necessary to develop a complete system description.  

In modern systems, especially where data is stored in RDBMS, the data flow paradigm is not always an especially good fit.  However, the context diagram, or level 0 DFD is still useful for describing the scope or boundary of the system in terms of data sources and sinks.