As mentioned in Basic Project Editing, the Project Timeline has two different components, the Primary Timeline, and the Secondary Timeline.
Items on the Secondary Timeline have some characteristics that differ from their Primary Timeline counterparts. While Primary Timeline items determine the length of the project, Secondary Timeline items do not. Also Secondary Timeline items are added to the the project starting at the current position of the playhead, and may overlap each other, while Primary Timeline items are added to the closest side of the currently viewed Primary Timeline item, and may not overlap, except during transitions, which are handled directly by KineMaster. Also, Secondary Timeline items do not have the one-second minimum-length restriction that is seen on the Primary Timeline.
There are seven different types of items that be placed on the Secondary Timeline. These are Media Layers, Effect Layers, Overlay Layers, Text Layers, Handwriting Layers, Music and Sound Effect Clips, and Voice-over Clips. Details about each are listed in separate articles.