Creating a submap works just like creating a new map and is done to edit an existing map in more detail. For example, to create the submap [test1] in the [test] map, right-click [test] and click [Add Map] in the popup window.
When the Create Map popup window appears, you can set [Map Name], [Map Size], [Tileset], [Change BGM], [Change BGS], [Single Play], [Instant Map], [Gravity], [Channels], and [Max Players] and then click [OK].
For this example, set [Map Name], [Map Size] and [Tileset] as in the below screenshot and click the [Ok] button.
This creates the map [test_1].
You can also create the map [test_2] in the same way.