The Split Module makes it possible to use two top modules or create more compact designs. Now you can create much more devices that combine for example LCD Module, Climate Module, PIR Module and many others. You can also use this module to create two stacks of modules and create more compact devices.
The Split Module also has connectors for two tag modules.
If you want to use a thinner module and you can spare the two tag module connectors you can use Compact Split Module. The Compact Split Module cannot be connected to the Battery Module directly so be mindful of that.