Declarative expressions are mentioned twice in the Defining Data Elements section:
in the Clipboard demo in the Inspecting Data Elements lesson (p 161 of Lesson Reference)
in the Review of Defining the Data Elements lesson (p 169 of Lesson Reference)
However, declarative expressions have not yet been defined anywhere I can find prior in the Lesson Reference, nor that I can recall in the lessons (I did not inspect all of the lesson transcripts, but I'm pretty sure of it).
Even though I'm sure they'll be fully defined later in the lessons, it might be worthwhile to give a brief definition of declarative expressions earlier in the lessons - perhaps something like, "Declarative expressions run automatically whenever a value changes."
Declare Expression is first mentioned in the "Introduction to Data Elements" lesson video "Demo: Create single-value properties to store data". The transcript can also be found in the Lesson Reference Document on page 135 as well:
"A calculated property is computed automatically by PRPC. We’ll learn how to compute a value with a particular type of rule called a Declare Expressions rule in an upcoming lesson."
The Declare Expressions is again mentioned on page 171 and then formally defined on page 420.