Solutions to Saturday Mac riddles 7

I hope that you enjoyed Saturday’s Mac Riddles, episode 7. Here are my solutions to them.

1: UTI (Uniform Type Identifier)
I’m not, as might appear, a problem with water (the common use of UTI = urinary tract infection), but a taxonomy (a hierarchical naming system developed by Apple) to replace gestural communication (one system it’s intended to replace is MIME, a type-naming system used over the Internet). What am I?

I bring an astronaut (astronaut = spaceman, the name of the APFS Space Manager) together with the grim figure of death (grim reaper, the reaper being the APFS mechanism to delete large objects over a period spanning several transactions) to progressively replace what your Macs have been running for 21 years (HFS+, which APFS replaces, was introduced as an upgrade from HFS in System 8.1, in 1998). What am I?

3: If a*a = a+1, and 1/a = a–1, what famous number is a? Answer: φ (phi) = 1.6180339887…, the Golden Ratio.
See Wikipedia for a detailed account.

I look forward to your putting alternative cases.