When you are adding or updating a Service Project, there is a field called "Log Service Project Hours" that has three options: "Online," On-site," and "Both." Each option has the following implications:
Online: When you select this option volunteers can log their own hours online using their volunteer login or a direct link which can be sent in a confirmation message. This gives the volunteer complete control to add hours to their account for this Service Project.
On-site: This option removes the ability for volunteers to log their own hours online, but instead provides the ability to record their hours using the Event/Project Sign-in Console (which can be accessed from the Main Dashboard on the admin side of the system, under the "Event/Project Management" heading.) This option will also allow volunteers to use Mobile Sign-in, if enabled for the event.
Both: This allows both the "Online" and "On-site" options to be enabled.
In all of these options, administrators are always permitted to add and delete hours for a volunteer in a Service Project.