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Fields hold descriptive data for each translation unit (TU) in a translation memory (TM). The TM maintains the values of some fields (system fields), such as Last modified on (date and time) for a TU. You can also create and maintain your own fields (custom fields). You apply penalties to matches based on the value of fields in the matching TU; for example, you can apply a penalty for matches with TUs that have not been used recently. You can filter displays of TUs by field values, for example to show only recently used TUs.
Fields are defined for each TM, but for server-based TMs they usually are created from field templates.
Try to use the same fields across all the TMs used for a language pair: if you have more than one TM in a TM sequence, only fields common to all the TMs are copied to the project TM, to be used during translation.