This page gives an example of what can happen when laying a minefield. Specifically, it covers information on when a new minefield will be created. It is the intention of this page to clarify the following information from the "Creation & Expansion of Minefields" section of the Minefields documentation.
To determine whether a new minefield is created or an existing minefield is expanded, the Host first needs to find the closest minefield center, where the minefield both is of the same type (normal or web) and has the same owner as the mines being created. If two qualifying minefields have centers precisely the same distance from the minelayer, the minefield with the lowest ID is used. If the vessel laying mines is currently within that minefield, it is expanded. Otherwise a new minefield is created.
There's a large minefield and a small minefield. There is also a starship in the large minefield, but just outside of the small minefield. The starship has the Lay Mines mission selected, with a Friendly Code of MD1.
Using the above description, we first find the distance to the centers of the relevant minefields. The center of the large minefield is 56.6 Light-Years from the starship, while the center of the small minefield is 38.4 Light-Years away. The center of the small minefield is closer, but the starship isn't in it. Based on the above description, we expect that a new minefield will be created.
The next turn, it can be seen that a new minefield has been created, as expected.
The order in which the initial two minefields were created isn't relevant to this example, but could be of interest to some people. The large minefield was initially created with 10 torpedoes, then the small minefield with 10 torpedoes, and finally, the large minefield was expanded with 20 additional torpedoes.