By value

A copy of the variable is available in
the function or event script. Any changes to its value affect the
copy only. The original variable in the calling script is not affected.

By reference

A pointer to the variable is passed to
the function or event script. Changes affect the original variable
in the calling script.

Read-only

The variable is available in the function
or event. Its value is treated as a constant—changes to
the variable are not allowed and cause a compiler error.

Read-only provides a performance advantage for some datatypes
because it does not create a copy of the data, as with by value.
Datatypes for which read-only provides a performance advantage are string, blob, date, time,
and DateTime.

For other datatypes, read-only provides documentation for other
developers by indicating something about the purpose of the argument.