STC, is short for Save to Central, (SWC is short for Synchronize with Central as Revit 2010 calls it now) is a common thing when you are working with Revit's Worksharing enabled. It is how we publish the work we have completed, return things we have borrowed and have the changes others have made placed in our Local File.
While working in your Local File take another look at the first row of toolbars and you'll find two SAVE Icons. The first is Save Local and the second is Save To Central for versions prior to Revit 2010.
These are the new buttons in Revit 2010 for Synchronize with Central as it is now called.
Under the File menu you'll find these, Save, Save As and Save to Central.
For Revit 2010 there is no File menu, there is the Application Menu and it does not have options for Synchronize with Central.
There IS a difference between the toolbar Save to Central (and Synchronize with Central) and the one found under the File menu. Most obvious is that this dialog box doesn't appear when you pick the toolbar button (or the Synchronize Now button).
This dialog only appears when you use File > Save To Central. Be sure to check all the boxes (except for Compact..., do that once at the end of the day) to return the stuff you borrowed, see I left the user created workset unchecked, setting a bad example!! Don't forget to check box the "Save the Local after Save to Central completes successfully" too!
With Revit 2010 a new option is described at the bottom of this dialog. Revit now Saves the local file first and then Saves it again after the SWC completes. This protects your local file before SWC and then saves the changes that occur in your local file after the SWC is finished. This process is important to keep your local file synchronized. It is important to check the option, which is checked by default now.
The other key difference between the toolbar and menu is the toolbar button does NOT return User Created worksets. It will return the Families, Project Standards and View worksets. Think of the toolbar button as the "quick" STC/SWC.