Conditional Revise option is Enabled by default since it is meant to optimize the synchronization process. Let’s see how it works.
When the option is Disabled, M2E Pro revises all the smallest changes to Product Quantity. Yet, updating of products with the high stock value (e.g. from 515 to 514) most likely has no practical effect, while may overwhelm M2E Pro servers. In this case, more important changes for the low stock products (e.g. from 2 to 1) may stay in a queue instead of being updated immediately. As a result, you may face overselling.
To avoid the issue, it is highly recommended to keep the Conditional Revise option enabled. For example, you can set Revise When Less or Equal to option to 5, and M2E Pro will revise your products in real-time only when Magento Quantity will be less or equal 5. This way, you will make sure all the important QTY changes are detected and revised on time.