Metadata is located under the META directory, it contains a XML file of the metadata and two JPEG thumbnails (small and large). It is very similar to the PS3 metadata (different format), except it doesn't support backgrounds, themed music, and video thumbnails.

• META directory: Subdirectory containing disc metadata and information.
• DL directory contains the DL XML metadata file
• dlmt_000.xml file: which contains the metadata for the whole disc and would include the title of the movie (or regarding content as a whole), and the JPEG/PNG image thumbnail.
• xxxxx.jpeg: large 640x320 title thumbnail icon.
• xxxxx.jpeg: small 416x240 title thumbnail icon.
• ES directory contains the ES XML metadata file
• esmt_000_xxxxx.xml: which includes the information for Title Scene Search, this is where the names of titles would be located along with keywords for searching them. Also, individual thumbnails for each title would be stored here.
• TN directory contains the TN XML metadata file
  • tnmt_000_xxxxx.xml: This is for Track/Chapter display where the names/text for individual tracks and chapters would be located.

Besides the title name, other metadata can be added such as names of chapters, songs, scenes, etc. You can also search for content on your Blu-ray player like a specific scene, actor, etc. (if available). BD music albums are capable of providing additional metadata that contains names of titles, tracks, songs, artist, genre, etc.

Thumbnails are in JPEG format and there are two sizes, small 416 x 240, and large 640 x 360.

Note: It is not confirmed or proven if PNG or GIF (static, animated, or transparent) is supported.

Note: Not all players have this feature, and not all BD titles have additional metadata, usually only the thumbnail and title name. Players such as PS3, PS4, PS5, Samsung H5100, and VLC support BD metadata, but not all BD players support or take advantage of its capabilities. The Sony BDP-Sxxx series supports the title name only, but not the thumbnail. Some players don't support metadata at all such as Xbox, LG, Samsung (older), Panasonic, and Oppo.

BD-ROMs are software applications, so they'll commonly have a thumbnail icon and title name like a normal app or desktop program.

<?xml version="1.0" encoding="UTF-8"?>
<disclib
	xmlns="urn:BDA:bdmv;disclib">
	<di:discinfo
		xmlns:di="urn:BDA:bdmv;discinfo">
		<di:title>
			<di:name>Aliens</di:name>
			<di:numSets>1</di:numSets>
			<di:setNumber>1</di:setNumber>
		</di:title>
		<di:description>
			<di:tableOfContents>
				<di:titleName titleNumber="1">Alien</di:titleName>
				<di:titleName titleNumber="2">Alien</di:titleName>
			</di:tableOfContents>
			<di:thumbnail href="A1_metadata_sml.jpg"/>
			<di:thumbnail href="A1_metadata.jpg"/>
		</di:description>
		<di:language>eng</di:language>
	</di:discinfo>
</disclib>

Sources

• Doom9 Forum Thread: PS3 Icons
• Doom9 Forums
• White Paper BD-ROM Version 3.2 Pages 25-29  - January 2018 (applies to normal BDs)
  
• Original research

Author(s) : Æ Firestone