question

@el C, I think, I answered your question. If you enhance a description of your question, then ask a new question. I have shown how to get items into storage system as you asked. Pallets are in my understanding only items.
Pallets can store boxes. If you have boxes with same attributes, then you assign the needed label value to pallets to store in the storage system. If you mix boxes with different attributes, it is up to you which label value you assign to the pallet!
If you want to know how you get boxes on pallets out of the rack, then yask a new question!
If you need a description from me, what I have done, then I will describe this in a comment.

I may help how a question can be answered, but I am not building models for users.

Hi @el C, my comment to the question how I come up with the code. Send To Port function returns a port number where the item has to go. I must end in my logic there. I have found in my years with FlexSim the command ipopno(..) at different source code logics in the past. It is mostly used in templates for tasksequences.
The other main part of my code depends on finding a slot. Slots are responsible in a storage system to get items into it.
After I have found a slot in a storage system, I can get all data to this slot by its class properties, but I must know which commands exists in FlexScript. There are two reference manuals - the older Command Reference and newer FlexScript Class Reference. FlexScript Class Reference has got classes for Warehousing. It’s uppermost class is Storage. From this class there are inherited sub classes like Slot. Class property storageObject returns a pointer to an object where the slot is. I can get with data the output port number of the queue to the object, that’s input port number is 1. That is what for the command ipopno(..) stands.
Writing source code is hard work, it is frustrating. There is so little success and much lesser progress. You see in my code still the primitive debug efforts I have taken. I use command print to understand what happens in my code. Each nuisance of change I print into the output console to understand how code works. If the code works, this means I have tested every change of code several times previously, I run the model a long time. There occur errors in the console. To get around them I have written the OnExit trigger source code for every involved rack object. Assigned slots aren’t disposed, if an item exits a rack. The item might return to its slot later. But if I want to populate the slot again, I must set the slot state to be unassigned. You don’t need the OnExit trigger code, if you let run the model only a short time, because a slot can store items until it is full. Full means all items assigned to this slots fits in by their size. An item assigned to a slot isn’t physically stored. It is just reserving the space to get stored. With progress of time in a model run there will be no space available to store further items into slots. This will let occur errors.
I hope I answered your question to my source code. Regards Jörg