The setting is in: Administration Center > Features > General.
Set Last Activity date of deals or contacts based on gives you the opportunity to set what modules will be used for the calculation of Last Activity field. The field is available for Deals, Projects, Companies, and Contacts lists. The field shows the last activity related to the item which is a journal, document, or email. The Last Activity field shows the date of this activity.
Click Set Last Activity date of deals or contacts based on to see the setting options.
You can select from Journal, Documents, and Emails modules. Based on what modules you select, the Last Activity field will be calculated. If you select no module, the Last Activity field will not be filled.
When you change the settings, be sure to click Save. For more information, go to Features.