What data decision you choose depends on what kind of information you are wanting to import.

If you are importing constituents to add them to the database, you might want Add only new constituents. Any existing constituents would not be changed in any way. 

If you are importing to add existing constituents to a group, you might want Overwrite blank fields. This ensures that you add the new data to the constituents, without changing any existing information. 

If you are wanting to update information for existing constituents, you might want Overwrite with non-blank values. This will replace data on the existing record with any new information in the import file.

If you are wanting to clear information from existing constituents, you would use Overwrite all fields. This will write blanks over existing data if the field in your import spreadsheet is blank. Use this option with care! 

The examples linked on the Data Decisions page of the import process are helpful and should be looked at carefully for further clarification about how each method works.