In the context of DisplayExpressions and agents, 'that' refers to note running the code. As expressions like find() change the meaning of 'this', 'that' provides access to the original value of this. From v7.5.0, 'that' is evaluated in stamps.
For instance, it makes it possible to to use an attribute value from the calling notes within a find() query in an expression. In such contexts, 'this' would apply to the note being queried by find() rather than the note calling find().
For example if Note A, has 'Note B' stored in $MyString and runs a rule (or some other action code) the following would fail to test each note in find() for a $MyString value of Note B:
find($MyString(this)=="Note B") Fail!
will test each note in scope of the find() query for the $MyString value stored in the calling note, i.e. Note A.