Length normalization impact.
Recommended values are around 0.75. Higher values increase the weight
that field length has on scoring. Setting this to 0 (not recommended)
means that the field length has no effect on scoring. Negative values are
invalid. Defaults to 0.7.
BM25+ frequency normalization lower bound (usually called δ).
Recommended values are between 0.5 and 1. Increasing this parameter
increases the minimum relevance of one occurrence of a search term
regardless of its (possibly very long) field length. Negative values are
invalid. Defaults to 0.5.
Term frequency saturation point.
Recommended values are between 1.2 and 2. Higher values increase the
difference in score between documents with higher and lower term
frequencies. Setting this to 0 or a negative value is invalid. Defaults
to 1.2
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Parameters of the BM25+ scoring algorithm. Customizing these is almost never necessary, and fine-tuning them requires an understanding of the BM25 scoring model.
Some information about BM25 (and BM25+) can be found at these links: