This is the default PicoSplit layout:
The layout is well thought out. I use it on a Mac with an English keyboard setting but you can also type umlauts with it. Although only a few keys are available, you can type all the usual characters. You even have two unassigned keys (16 and 39) at your disposal.
When we talk about a keyboard layout, we often mean two things: The arrangement of keys on a keyboard and the key assignment.
In the PicoSplit firmware the layout term is used for the key assignment.