Passing arguments to functions and events – PB Docs 2019

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.