For stock reported in Inventory Planner from a connected FBA store, the amount of stock shown is based on the available units for sale from Amazon. 

There are three reserved quantities that can affect the stock quantity available: Customer Order, FC (fulfillment center) Transfer, and FC Processing. Inventory Planner reports the number of units available for sale from the Amazon report. Typically this includes FC Transfers but excludes FC Processing and Customer Order. Amazon notes that from time to time, that some units in transfer from one warehouse to another can briefly be in both the FC Transfer and FB Processing categories.