If you export some data, then edit it with purpose of importing it back, please observe the following recommendations:
An ID will be allocated to each contact in ACT! You are not able to call up this ID, however, it exists.
All activities, histories, etc. allocated to the contact will have the same ID. This so-called ID is important for you if you want to import your data into a new database or a new program after export.
If selecting 'Check with IDs' the IDs will also be saved in the file. During import the IDs will be imported as well and the affiliation of data will remain.