In the diagram, what do the symbols with '+' and '-' inside rectangles represent?

Analysis of Symbols:

In the context of electrical circuits and fields:

• The symbol with a '+' inside a rectangle (or circle) typically represents the positive terminal of a voltage source (like a battery) or a region of positive electric potential.
• The symbol with a '-' inside a rectangle (or circle) typically represents the negative terminal of a voltage source or a region of negative electric potential.

Conclusion: The symbols with '+' and '-' inside rectangles represent the positive and negative terminals of a voltage source, respectively.