A DVD-compliant disc contains files necessary for playback of DVD-Video, and is made up of one Video Manager (VMG) and multiple Video Title Sets (VTS). The VMG is composed of up to three files:
1. VIDEO_TS.IFO consists of control and playback information for the entire DVD. Locations of all Video Title Sets on the disc are stored in this file. The first-play PGC is also stored in this file. VIDEO_TS.IFO is known as VMGI (The Video Manager Information file). This file is required to be present on a
DVD-compliant disc.
2. VIDEO_TS.VOB contains the contents required for the title menu. The title menu is usually designed to get access to video titles present on other VTS, and is displayed when the "Title" button on the remote control is pressed. Short video clips without audio such as FBI warnings, copyright messages are usually stored in this file of the VMG. This file is not required and may or may not be present on a DVD-compliant disc.
3. VIDEO_TS.BUP is an identical copy of VIDEO_TS.IFO. If the DVD player cannot read information from the VIDEO_TS.IFO, it will attempt to get the required information from this backup file. This file is required to be present on a DVD-compliant disc. The other numbered VTS have the same structure with an IFO, a menu VOB (if there is a menu, there doesn't need to be one.) then the rest of the Video Object (VOB) files with the audio & Video data for the Title(s) in the VTS [There could be up to 99 titles in each VTS.]
Each VTS must contain video & audio with matching parameters.
For a DVD to have video with different aspect ratio, or image size, or a few parameters that are different; then each kind would need to be in its own VTS.
That's basically what the file structure of a DVD is about.