Within the calendar app you can have several different calendars. The default ones are n900 and Private. I've always assumed Private is for events you don't want to sync and don't use it. You can set different display colours for each and decide which are shown. When you tap Use xxx the events in the file are imported into the database for the calendar xxx. I use a seperate calendar for Holidays as it allows me to colour code them and delete them en masse without affecting my personal events.