Here's my point of view:

• A statement is the basic command that can be sent on its own to the database engine. All statements are listed as such in the docs sop they are clearly defined.

• Many statements consist of several building blocks called clauses - some are optional (like the WHERE, GROUP BY or ORDER BY clauses in a SELECT statement), others are mandatory (the SELECT-list clause itself e.g.), so they are not necessarily switches. Not all statements are made up of several clauses, say a COMMIT statement has only one building block. I don't know whether that is technically also called a clause or not, but I'm sure one would not use that term here.

• As you state, functions can relate to different things, related to the SQL standard or to vendor-specific builtin function, or to user defined functions (UDFs or stored functions created via CREATE FUNCTION). Sometimes functions and procedures are treated together, and they are contained in the same system table...). I guess the different prefixes of the builtin system functions and procedures have to do with the fact whether they are truly SQL Anywhere-specific (often named as "sa_") or shared with other SQL dialects like ASE/MS SQL (often "sp_" or "xp_" for extented procedures) but that's primarily my own guess.