Which quantity wins
Quantities move from Odoo to Salla only, by two different mechanisms depending on hosting.
One direction only
Odoo is the source of truth for quantity. The connector reads the available quantity in Odoo and writes it into Salla. The reverse never happens: editing a quantity by hand in the Salla dashboard does not reach Odoo, and will be overwritten at the next sync.
Mechanism one: the Tecfy addon
On Odoo.sh hosting: the Tecfy addon installed inside Odoo watches stock moves and calls the connector service the moment a quantity changes, which updates Salla. The update is near-instant.
Mechanism two: a scheduled action
On Odoo Online hosting: no addon can be installed, so the connector builds two scheduled actions inside Odoo that run every twenty minutes: the first prepares a quantity snapshot, the second notifies the connector to push it to Salla. The update is not instant.
Which locations are counted
The locations counted toward the quantity sent to Salla are set on the company record inside Odoo. Choose more than one and their quantities are added together. An item sitting in a location you did not select shows no quantity in Salla.
An unmapped product gets no quantity
Exactly as with orders, the connector cannot update the quantity of an item whose Salla counterpart it does not know. See "How a product is matched".