Field extensions tell Import which columns go together to add certain data to The Raiser's Edge. The same number should be selected for groups of fields that add a specific piece of information.

The Raiser's Edge will present the extension column when appropriate. If they are not required or are not an option for the field, then the column will either not be present or the option to select a number is not available. (For more information, please refer to I cannot edit or change or find the Extensions column when mapping import fields.)

Field extensions are often used when there is a "one to many" aspect of the Import. Fields that just have one value, such as Constituent ID, Last Name, Org Name, will not have an Extension option. However, if more than one of a field or record  can be added in Import, then extensions will be needed.

A common scenario for extensions is when importing phones and emails:

It takes multiple columns to add one phone. Each column with information about the same phone should have the same extension so Import knows they go together to add that value. If there is a second phone on the same row of data, then the next group of  columns with the second phone's data should have the same extension between them but different from the first group of Phone fields.

Here is how mapping two phones in a constituent would appear for The Raiser's Edge 7.94 and higher. Note how the grouped phone fields have the same extensions. The information in those three columns should be the data for one phone/email:
Sample mapping of extensions for phones in RE 7.94+

And here is how mapping two phones in a constituent would appear for The Raiser's Edge 7.93 and earlier. Note how the grouped phone fields have the same extensions. The information in those four  columns should be the data for one phone/email:
Sample mapping of extensions for phones in RE 7.93 and earlier
 

Other imports that may require extensions are Attributes, Addresses, and Relationships. For specific details on an import, search the Knowledgebase or refer to the Import Reference List.