Just wondering why only left and right in the SplitDirection Enum

Hi, this is my first post on cesium community, excuse my poor English.

I would like to inquire as to why the ‘SplitDirection’ enum only has ‘left’ and ‘right’ as possible values? Recently, I’ve been trying to implement a comparison feature for TIF layers, and while the ‘left’ and ‘right’ options enable me to perform horizontal comparisons, I seem to be stuck when it comes to vertical comparison. Can you explain the reasons for limiting the enum to these two values? If I want to accomplish a top-to-bottom comparison, is my only option to extend ‘SplitDirection’ with ‘top’ and ‘bottom’ values myself? Thank you.