Translation Memory fields allow you to further describe the translation units (TU) that are stored in your translation memory. There are two types of fields:
System fields: These are fields which are automatically created for all translation memories. For example, there are system fields that hold the date on which the TU was added and the name of the person who added it.
Custom fields: These are fields that were added to the translation memory when it was created or when it was later edited. For example, if you want to store a project code for each TU, you could create a field called Project Code and assign a value to the field when the TU is added to the translation memory.
You can use both custom and system fields in filters.
Overview: Creating New Translation Memories