Above the 75th percentile
Eight digits and up is no longer recall, it is a holding method: at that length the run stops fitting in one pass, and it has to be cut into groups of three or four and rehearsed. The technique is called chunking, it is trainable, and on this scale it buys more than raw capacity does. Hence the main caveat about a high score — it describes your strategy at least as much as your memory. People who routinely hold codes, phone numbers and totals in their head arrive here with the technique already built. What the number does not promise is that you will remember a name or where you left your keys: that is a different memory, and digit span does not carry over to it.
Middle: 25–75
Five to eight digits is the most crowded part of the scale, and it is the range Miller described in 1956 as seven plus or minus two. At this length the run usually still holds whole, without being cut into groups — you simply repeat it to yourself until you can type it. It breaks predictably: not on the last digit but somewhere in the middle, because the start and the end of a run are remembered better than its middle. The practical takeaway from a mid score is narrow: gains here come not from training memory in general but from one specific habit — cutting a long run into groups of three.
Below the 25th percentile
Four or five digits is a short run, and the first thing worth checking is the conditions, not the memory. Silent rehearsal decides almost everything here: if the inner repetition was interrupted — lyrics, someone talking, a notification mid-run — the digits are gone in a second. This is also one session on one device: short sleep, coffee on an empty stomach and an unfamiliar keyboard move the result further than a whole band of this scale. The number has no clinical meaning whatsoever, and it does not describe your memory in general — only how many digits survived one pass in one particular minute.