Unit maximum mass load seems to be capped at 1200.
To test it, read my repro steps to fill your inventory and then this code will work :

    (load player == 1) && (loadAbs == 1200)

At this point, the engine considers the player "full" and won't allow neither right clicking on an item to put it in inventory nor script commands as :

player addItem "blabla"

Also the various commands canAdd(ItemToXXX) will return false.

Problem is, the white bar at the bottom of the inventory GUI won't be full, and if you have equipments with lots of capacity, you might also have some free space in your backpack.

From here, since right clicking and classic commands won't work, you can drag and drop the item, or use commands like addMagazineCargoGlobal on the object returned by (backpackContainer player) which absolutely won't care about the container's capacity.
An excellent example is found in the A3Wasteland mission source code at https://github.com/A3Wasteland/ArmA3_Wasteland.Altis/blob/master/client/functions/fn_forceAddItem.sqf

I checked, and I seem to reach a definitive maximum (where the bottom white bar is full and drag & drop won't work) of :

    loadAbs == 1324

In conclusion, I guess it would be cool to first resolve this bug if it's one, then also add a command to retrieve the maximum mass load of a player, so the scripter can finally cover all corner cases of adding an item to inventory and correctly handle eventual failures.
We could then say goodbye to workarounds àla fn_forceAddItem.sqf found in A3Wasteland :)
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