Absolutely hellish indeed. I had to write a parser for some kind of custom IATA PAXLST [0] (passenger list) that was coming from an ancient automated email system and I was dumbstruck by the weirdness of it all while reading the documentation.

    UNB+UNOA:4+SWAIR+NZCS+080708:0545+000000011'
    UNG+PAXLST+SWAIR+ NZCS+080708:0545+11+UN+D:05B'
    UNH+SWCA749+PAXLST:D:05B:UN:IATA'
    BGM+745'
    NAD+MS+++WILLIAMS:JANE '
    COM+41 22 797 2025:TE+41 22 788 4689:FX'
    TDT+20+SW679'
    LOC+125+GVA'
    DTM+189:020322'
    LOC+87+BOS'
    DTM+232:020322’
    DTM+189:020322'
    LOC+92+ATL'
    DTM+232:020322'
    DTM+189:020323'
    LOC+130+SFO’
    DTM+232:020323'
    NAD+DDU+++WINDSOR:ELIZABETH:R'
    ATT+2++F'
    DTM+329:720623'
    LOC+178+NCE'
    LOC+22+ATL'
    LOC+179+MEX'
    LOC+174+FRA’
    NAT+2+CHE'
    RFF+AVF:RXDWWH’
    DOC+P+564SBB41'
    DTM+36+051215'
    LOC+91+FRA'
    DOC+V+VV258DD7'
    DTM+182+021212'
    DTM+36+0421201'
    LOC+91+:::MARSEILLES’
    CNT+42:1'
    UNT+33+SWCA749'
    UNE+1+11'
    UNZ+1+000000011'

[0] https://www.iata.org/contentassets/18a5fdb2dc144d619a8c10dc1...