MoboMath supports multiple independent math expressions, each in its own region.
There is always a current region, which by default has a yellow background; other regions have grey backgrounds. Many menu operations apply to the current region.
To select a new current region, just tap in it away from all current characters. If you tap too close to the text, MoboMath may interpret it as request for an alternate recognition result.
When you write in the MoboMath window you can create a new region by writing well away from the existing regions, or you can modify or add to existing regions.
Multiple regions give you useful ways to edit expressions. You can drag a subexpression in one region onto a character or selection in another region to replace it, or drag the subexpression to an empty area in another region to insert it. To replace an entire subexpression, be sure to select it first.
If you are building a very complex expression, it's often easiest to write parts of the expression in different regions, and then put the expression together by dragging the parts into place.