"Containers under management" is defined as the total number of containers currently leased in/out via xChange under your account.
Example: You have leased out a total of 20 containers to 3 partners and at the same time leased in 7 containers from another partner. This brings your total containers under management to 20 + 7 = 27.
Relevant for the calculation of your pricing tier is number of containers under management that you have had in a given month.
It is important to differentiate this from your credit limit which defines how many containers you are currently allowed to use (lease in) at any given point in time. Relevant for the credit limit is your current net exposure which is calculated as "containers used" - "containers supplied". In the example above, your net exposure would be 20 - 7 = 13.