Using the documentation related to
Importing TM content we can see the "Discarded translation units" are those which have not been imported. This could be for various reasons. When importing a TM the user is presented with General and Advanced Options and must select options to instruct the application on how to import the TU's and what to do under certain situations (e.g., when the target segments differ, how to handle unknown fields, and how to import based on status). These are possible scenarios that may lead to a Discarded translation units. Also, if TU already exists in the TM, it is ignored.
