Having related meters means you could have 2 separate electricity meters, or one meter with 2 supply points.
A supply point is how suppliers provide and charge for electricity. A typical meter has just one.
They go by different names depending on supplier
They can be called dynamically teleswitched energy meters, exotic meters, Total Heat meters, Total Control meters, and Weathercall meters.
This was done in the past to encourage a move to electrical heating
Properties had related meters installed to allow a different rate to be charged for heating separate from other electricity use. In most properties, electricity goes from a meter to a fusebox, which then separates it into circuits for heating, lighting, and powering other things. With related meters, electricity will go from the heating supply point directly to a circuit with its own register. It was originally set up this way to save time and money during installation.