When you load the inventory you assign it to either Master agent, Distributor, Retailer or employee. This setting here determines if the employee have the access to the inventory assigned. Below are the details.
If the Inventory User Level access is set to Master then user will have the access to all the inventory under that master agent.
If the Inventory User Level access is set to Distributor then user will have the access to all the inventory under that Distributor.
If the Inventory User Level access is set to Retailer then user will have the access to all the inventory under that Retailer.
If the Inventory User Level access is set to Employee then user will have the access to the inventory under itself.