Today I will show the setup that determines when an inventory organization is actually Master, in which you can have your settings defined at its level a by the hierarchy will serve the other organizations.
There is no specific configuration that determines which organization is Master or not, so that it is considered, you must follow the steps below.
Go to Inventory -> Configure -> Organizations -> Organizations -> Create an Inventory organization.
Repeat the steps and create another organization.
Go to the parameters of the Inventory Master organization and configure that the Master organization is the one in which you are assigning.
If the company has 10 units you can set up the same Master organization for all inventory organizations or set up for each one a Master organization