Marked time records normally represent a subset of a document's time records.  This subset can be sorted, printed, and exported, giving you valuable information.  

All totals such as the Total Elapsed and Total Charge are calculated either from all time records or from marked time records.  To tell how the totals are calculated on a document, look for the Total Marked Indicator.  It is the black dot just below the Status Light:

When it's visible, the totals are calculated from marked time records.  If it's not visible, the totals are calculated from all time records.  Switch between the two by clicking on the Totals icon in the Tool Bar (4th button from the left), or by selecting “Total Marked Time” or “Total All Time” from the Time Menu.

Mark New Records 
Available in the Defaults window, under the Time menu (see the chapter “Default Time Record”).  When this item is checked, all new time records (created by clicking on the Start button) are marked.  This could allow you to total records for a particular session. At the end of the session you could export just these new time records to a database program.

Mark Special
This command (under the Time menu or click the Mark Special button) allows you to mark all records for a job and/or date range.