The book's section on Declarative Pipeline Syntax says:

• Blocks must only consist of Sections, Directives, Steps, or assignment statements.

There is no elaboration on what constitutes a valid assignment statement. Perhaps that means that assignments "follow the same rules as Groovy’s syntax," but given that there is not a section in that Groovy document specifically about assignment, that leaves a lot of uncertainty.

• Where can assignment statements appear?
• What can be used on the right hand side of an assignment?
• What is the scope of a variable?
• Must variables be declared with `def` or `var` prior to assignment?