XSPF is a format to enable sharing, AKA universality. It does this by defining a list of metadata fields to be used for resolving each track in the local context of the listener.
The UPF "ids" field maps to the XSPF "identifier" field:
http://xspf.org/xspf-v1.html#rfc.section.4.1.1.2.14.1.1.1.2
XSPF is not M3U in any way. From the spec:
http://xspf.org/xspf-v1.html#rfc.section.3.4
3.4 Content resolver
On a surface level you can use XSPF like any other playlist format. Drop a bunch of filenames into an XSPF document, prepend "file://" to each, and you're ready to go. Under the surface there is much more.
The guiding design principle was to separate the functionality of a catalog of files from the functionality of a list of songs. Most software music players on the PC have some sort of cache for file information. This cache stores a list, or catalog, of available files and metadata from ID3 tags and other sources. XSPF is not a catalog format. XSPF exists only to say which songs to play. Almost everything in XSPF is for the purpose of answering the question which resource, rather than the question what is this resource.
If XSPF is not a catalog format, what is it? XSPF is an intermediate format. We expected a new kind of software called a content resolver to do the job of converting XSPF to a plain old list of files or URIs. A content resolver would be smart enough to keep your playlists from breaking when you move your media from /oggs to /music/ogg. It would be able to figure out that a playlist entry by the artist "Hank Williams" with the title "Your Cheating Heart" could be satisfied by the file /vorbis/hankwilliams/yourcheatingheart.ogg. It might even know how to query the iTunes music store or another online provider to locate and download a missing song.
The content resolver maintains the catalog of your songs in whatever format it prefers. It might use a flatfile, a file in the Berkeley DB format, or a SQL database. It might use only ID3 metadata, but it might also know how to query MusicBrainz or another metadata service.
All XSPF user agents are content resolvers, in that they have complete leeway to turn the contents of a track element into a specific set of bytes.
3.5 Fuzzy names
Any given track can be identified in a number of ways. We provided means for absolute identifiers like URIs, filesystem paths and secure hashes, but also for query-based identifiers — free text fields like artist and work title and numeric fields for song length, all of which together should be enough for a good content resolver to turn into files.