Heliographic codes

[Lothia]

Ave Civitas,

I am reading John Rogan's "Roman Provincial Administration"
In his chapter "The Governor in Office" he writes that:
A signalling system assisted communications: the heliograph in sunnier provinces' fires and smoke in duller climes.
... relay stations would be required.
They were only suitable for brief commands, requests, or information.
It would have been risky to send messages en claire: Official codes would be necessary.

I cannot imagine anything but codes being used to transmit anything via heliograph.
So, my question is, does anyone know what those codes were? Is there a source I might be able to access?

as always,
Thanks..