The rules that matter
Keep it short. A list you can hold in your head produces consistent coding; a list of twenty options produces two people classifying the same loss differently. Make the options mutually exclusive. If two causes could reasonably apply to the same deal, the boundary between them is not clear enough. Separate the competitor from the reason. Which rival won belongs in its own field, otherwise the list grows by one option every time a new name appears. Give no decision its own category. It is usually the largest single group and it is the one most often hidden inside other.
A workable starting set
Lost to a competitor. No decision or status quo retained. No budget available. Missing capability. Wrong fit or poor qualification. Timing. Built in-house. Seven options, each with a clear meaning, plus a free-text note and a competitor field. Most teams find this covers the great majority of losses without forcing anyone to guess.
A worked example of loss analysis (illustrative)
Suppose a team closes 60 lost deals in a quarter and the distribution is: 24 no decision, 14 lost to a competitor, 9 missing capability, 7 no budget, 4 timing, 2 wrong fit. Read alone, the headline is that the biggest competitor is inaction. Now add the stage at which each was lost. If most of the 24 no-decision losses occurred after a proposal, the problem is that the case for change was never established, and it belongs in discovery. If most occurred before the second meeting, the problem is targeting or the opening conversation. Add source next: if the 9 missing-capability losses all came from one channel, that channel is attracting a segment the product does not currently serve. Three dimensions turn a tally into three different projects, each with a different owner.