Which numeric caption limits (cps, words per minute, characters per line, lines, min/max duration, gap) do Netflix, BBC and the FCC state, and how do I pick by context?

Problem. Sources give different numbers and no single 'best'. Conditions. Choose the profile by delivery context. Method. Documented figures (page dates in bibliography): NETFLIX (streaming, English): 42 characters per line, max 2 lines, reading speed up to 20 cps adult / 17 cps children, min duration 5/6 s (20 frames at 24 fps), max 7 s, minimum 2-frame gap (all frame rates). BBC (UK broadcast and online): 160-180 words per minute (0.33-0.375 s/word), target minimum about 0.3 s per word, 37 characters per line for broadcast Teletext, online lines limited to 68% of 16:9 width (90% for 4:3, 1:1 and 9:16), max 2 lines landscape or square, 3 lines recommended for 9:16, a deliberate gap must be at least 1 s (preferably 1.5 s). FCC 47 CFR 79.1(j): states quality criteria (accuracy, synchronicity, completeness, placement) with no cps, line or duration numbers; only a live-caption accuracy metric formula (correct words / total words). Example. A 6-word line needs about 1.8-2.0 s at BBC rates; as characters it is about 30 characters, which at Netflix 20 cps needs 1.5 s. Failure modes. Mixing units: BBC counts words per minute and Netflix characters per second, so convert only as an approximation. Netflix pages disagree internally: the General Requirements page says min duration 5/6 s (20 frames at 24 fps) while the Timing page says 20 frames 'or 4/5 sec'; 20 frames at 24 fps is 5/6 s, so treat the frame count as the rule. Limitations. Netflix gaps are frame-based (2 frames) while BBC gaps are about a second; they are not interchangeable because the BBC figure is a pause-gap rule, not a flash-avoidance rule. Version/date. Netflix pages undated (General Requirements change log to 2022-10-07); BBC page version 1 from Sept 2016 plus updates; FCC LII copy. Inference: for a platform with no stated rule (YouTube, social), a defensible default is 42 characters per line, 2 lines (3 for vertical), at most 17-20 cps, min about 0.8 s, max 7 s, but no source sets this for YouTube. Curator qualifications: All cited documentation excerpts located on the live pages by curator GET; no video sources. Limitations: Sources are 2016-2022 era pages; Netflix pages may have been revised without a visible date. DCMP Captioning Key (reading rates) was unreadable (HTTP 403) so is not compared. Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

{
  "citations": [
    {
      "url": "https://partnerhelp.netflixstudios.com/hc/en-us/articles/215758617-Timed-Text-Style-Guide-General-Requirements",
      "note": "1. Duration; I.2 and I.14; I.14 Reading Speed Limits; Introduction; 3: Minimum duration; 4 Timing; 3.1 Line length; 3.3 Number of lines; 4.5 Gaps; (j)(2)(ii) Synchronicity; researcher reasoning"
    },
    {
      "url": "https://partnerhelp.netflixstudios.com/hc/en-us/articles/217350977-English-Timed-Text-Style-Guide",
      "note": "1. Duration; I.2 and I.14; I.14 Reading Speed Limits; Introduction; 3: Minimum duration; 4 Timing; 3.1 Line length; 3.3 Number of lines; 4.5 Gaps; (j)(2)(ii) Synchronicity; researcher reasoning"
    },
    {
      "url": "https://partnerhelp.netflixstudios.com/hc/en-us/articles/360051554394-Timed-Text-Style-Guide-Subtitle-Timing-Guidelines",
      "note": "1. Duration; I.2 and I.14; I.14 Reading Speed Limits; Introduction; 3: Minimum duration; 4 Timing; 3.1 Line length; 3.3 Number of lines; 4.5 Gaps; (j)(2)(ii) Synchronicity; researcher reasoning"
    },
    {
      "url": "https://www.bbc.co.uk/accessibility/forproducts/guides/subtitles",
      "note": "1. Duration; I.2 and I.14; I.14 Reading Speed Limits; Introduction; 3: Minimum duration; 4 Timing; 3.1 Line length; 3.3 Number of lines; 4.5 Gaps; (j)(2)(ii) Synchronicity; researcher reasoning"
    },
    {
      "url": "https://www.law.cornell.edu/cfr/text/47/79.1",
      "note": "1. Duration; I.2 and I.14; I.14 Reading Speed Limits; Introduction; 3: Minimum duration; 4 Timing; 3.1 Line length; 3.3 Number of lines; 4.5 Gaps; (j)(2)(ii) Synchronicity; researcher reasoning"
    }
  ]
}