The prime uses phrases like "open sentence"...

[Han]

From Mathematical Proofs, 3rd ed. (page 38):

Quote:In general, an open sentence is a declarative sentence that contains one or more variables, each variable representing a value in some pre described set, called the domain of the variable, and which becomes a statement when values from their respective domains are substituted for these values.

A statement is simply a declarative sentence or assertion that is true or false (but not both).

These are terms typically introduced in a formal logic class. In the case of the HP Prime, I only know of the use of open sentence in the Advanced Graphing app, since it indeed creates a plot-point based on the truth value of the open sentence after substation of X and Y values into the sentence.