Query: Expression.add() versus AND

Can someone please briefly explain what is Expression.add()?

Add is math (i.e. 2 + 2 = 4). And is logic (i.e. true && false == false). Are the documentation comments unclear about this (e.g. this page)

Oh! There are .net docs! And they are nicer, clearer than the Java docs. (Never occurred to me to check the .net docs.)
The JAVA docs are not as clear in general, eg in this case it misses a key word: mathematically adds. (Just - adds one expression to another)
Glad to find another trove of docs.