How do I leave some fields in my source file unmapped?

If you are using a data map A file that contains information about how data from a source system corresponds to data in Microsoft Dynamics CRM Online. for your import, you can specify that you want to ignore a column by selecting the column and then clicking Ignore. More information: Work with Data Maps

If you are using automatic mapping Importing a data file where the column headings exactly match the Microsoft Dynamics CRM Online display names. No data map is necessary. With automatic mapping, you can map columns that establish a relationship with existing Microsoft Dynamics CRM Online records., you can click the Edit button on the Review Mapping Summary page in the Import Data Wizard.

You also have two options:

• Remove columns from your source data that you do not need in Microsoft Dynamics CRM Online. The easiest way to do this is to make a copy of your source file, and open it with Microsoft Office Excel. Delete unused columns, and then save the file as a comma-separated value (CSV) file A data file with a .csv file extension. Typically a CSV file consists of fields and records, stored as text, in which the fields are separated from one other by commas..
• Customize the entity in Microsoft Dynamics CRM Online and add new attributes so that there is a destination for each source field in Microsoft Dynamics CRM Online.