This field can be used to upload a TMX file to the NEC-TM resource, that you create. Basically this means, that this TMX will be imported into NEC-TM, labeled with the NEC-TM categories (tags) that you select when you create the language resource. Also assigned to the imported segments will be the categories / tags, that are defined in the Zf_configuration table of translate5 to always be assigned to all segments that are coming from the current translate5 instance.
The same will happen, if you add/upload a TMX file to an already existing NEC-TM language resource in translate5.
Can this field be used to upload a TMX that will be searched before going to NEC-TM?
If that's no the use, do you know of any 'simple' way to add a single TMX as language resource other than using a TMServer?