Hi Paul,
In planning book, you can use below button to change peiodicity to display the data, however it only displays data in liveCache - in which disaggregation is done in liveCache. However in period split function, splitting is done using ABAP coding, so it does not follow the logic as in planning book. This means you cannot simulate period split in planning book, but you can use the simulation function in the period split screen.
Hope this clarifies your question.
BR, Ada