The wording of the text messages the system sends by itself. Two groups today: store orders (confirmation, shipping, cancellation) and bookings (confirmation, reminder, cancellation, reschedule, payment link).
The segment counter is optimistic for Persian
The counter under the box treats every 160 characters as one segment. That is correct for Latin text.
Persian messages are sent in an encoding whose segments hold about 70 characters. So Persian text typically uses more than twice the segments the counter reports.
Since cost is charged per segment, that difference lands directly on the SMS bill. Keep the text short.
A site's template sits on top of the platform's
Each template has a default version for the whole installation. Write one for the active site and only that site uses it; the others stay on the default.
Resetting removes the site's override and returns it to the default — it does not blank the text.
Each language is separate
As with email templates, every language is its own version, and the language used when sending comes from the message itself rather than the one you are editing.
Variables are a fixed list
Only the variables shown beside the editor are substituted. Anything else is sent to the recipient literally — which in a text message both looks wrong and costs segments.
Writing a template does not switch SMS on
Sending has to be enabled separately in that module's own settings — Store → Settings for order texts, Booking → Settings for booking texts — and the site's SMS service must be configured with a line that can send ordinary messages (a line limited to sign-in codes cannot). A written template sends nothing on its own.