What is 7/0.2 wire?

7/02 wire is a wire made of 7 strands of 0.2mm wire.

Stranded wire has the advantage of being flexible while solid wire is more difficult to bend.

7/02 size usually used for model railways for most connections. For example, this would be used as a link wire for extending the power in your layout. 

With larger layouts there is more chance of having dead spots in your track due to the power now being able to get to the furthest reaches of your layout, by adding link wires you are ensuring that the power is continuous, allowing the smooth constant running of your engines .

7/02 wire is also suitable for points motors and lights.

If you are running a bus wire, you may need to consider heavier wire.