There was once an 'improved' version of MP3 called MP3Pro which included something called 'spectral band replication - SBR' (don't ask!). Ages ago I spent some time getting an MP3Pro decoder to work because there was an online radio station called Whole Wheat Radio which used it.

The key line in the MP3Pro_wikipedia_entry is

Thomson Multimedia licensed the technology and used it to extend the MP3 format, for which they held patents, hoping to also extend its profitable lifetime.

SBR is used in AAC, a more modern and 'better' compression method (which is also proprietary - and whose patents have not yet expired). AAC is favoured by Apple. M4A files usually contain AAC-compressed audio.

Non-proprietary formats are good if you want to be able to access your recordings in the future. So you'd think I'd have digitised my vinyl into the open, free, ogg-vorbis format. But I didn't because most of my portable devices couldn't play it.

Not all MP3 encoders will be equally good. You're likely to get better results, and have more choice of output quality, from an encoder on a computer such as LAME than one in a small device.