It would be useful for freight transport in general (and for the personal inventory) if any inventory slot and in particular the slots of Freight Car and vehicles like the Truck and Tractor could be locked to a specific item type.
Usage: Simple usage and well known for Factorio users: Just place a stack in a slot and press the middle mouse button on it. This locks the item type, which can be stored there. Use SHIFT+MiddleMouseClick to clear the assignment and reopen it for everything.
UI: The locked slots always display the items icon even if there is nothing in there (0 is displayed) and the background is in a different color to indicate that this is a locked slot and to distinguish the stacks from normal stacks.
Behaviour: Locked slots are from than on only available for items of this type. Drag and Drop from or to this slots is only allowed for this item type. Freight Stations and Truck Station can unload from these slots, but for loading only when the item is available. Locked slots are also exempt from inventory sorting and stay where they were assigned to.
Applications / Examples
Mixed Freight Transport - This means for example that trains which stop at multiple stations or at one station with a mixed inventory that only available slots are filled on loading.
The train can stop at multiple stations even if there are different items to load or only for another car in train. The locked slots will never be filled with something else.
Players could assign the hole car with all ressources and products of one production cycle in the right quantities according to the recipe and transport everything in one car and need only one Freight Station Module.
Mixed Storage - In Stations (Truck/Freigth Train) this slot locking could ensure that only certain items are stored and if which quantities at maximum. In normal storages the same, the player can choose how much of any item is stored at maximum (number of slots/stacks). Of course there might be more to this, e.g. how to handle item which are too much or not the right type, e.g. passthrough to the exit or something like that.
Stations: Item Type Selection for Unloading - Of course this system would work even better, if at unloading stations the players could set which item type(s) to unload (Whitelist), maybe which one should not be unloaded (Blacklist). Than this system could be made even more useful. Maybe the same for the loading.
Improvements to Smart and Programmable Splitter - Mixed Transports are currently a little problematic because the Smart Splitter cannot reliable filter items with overflow handling, which is must have for this kind of transportation to be able to sort mixed belts. Of course it can currently handle guranteed filtering of one item type per exit already but not overflow and not multiple exits. Please see the following suggestion for that.