(This tip applies to Skyline-Daily 184.108.40.20657 and later.)
When enabled, the audit log will keep track of all changes that are made to the current document. The audit log is stored as a separate file (.skyl), alongside with the skyline document.
The audit log can be accessed from the View menu. The audit log is displayed in a grid, similar to the document grid. In the top right corner audit logging can be enabled or disabled.
For new documents, audit logging is enabled by default.
Full details can be found here (PDF).