Mapper Key seems to be arbitrary

Question

When coding a mapping the key used does not seem to matter. Is there a case where the value matters. PickerBase can be any string and it still works.

    PickerHandler.Mapper.AppendToMapping("PickerBase", (handler, view) =>  

Answer 1

Correct. In general it's arbitrary - you can set it to anything that makes sense for your application. The name only matters if you're trying to modify a specific known mapping that you or somebody else will try to query for later.

See Customize a Control with a Mapper

A string-based key. When modifying one of the mappings provided by .NET MAUI, the key used by .NET MAUI must be specified. The key values used by .NET MAUI control mappings are based on interface and property names, for example nameof(IEntry.IsPassword). The interfaces, and their properties, that abstract each cross-platform control can be found here. Otherwise, this key can be an arbitrary value that doesn't have to correspond to the name of a property exposed by a type. For example, MyCustomization can be specified as a key, with any native control modification being performed as the customization.