DNA matches are a combination of science and statistics. Whenever statistics are involved, we are dealing with probabilities. The probability is that someone with whom you share a largest match of at least 35 cM is going to be related, but you will not always be able to prove it. That is because most of us who are descended from eastern European Jews cannot trace that descent back before our ancestors came to western Europe or North America, either because we do not know the town in which they lived in easern Europe, or because the records there were destroyed, or our ancestors neglected to register.
Most genealogists have some cut-off below which they do not both with a match. People have told me that they need a largest match block of 20 cMs plus a second block of at least 10 cMs to make the match worth investigating. I am likely to investigate any match of at least 25 cMs, but I recognize that at the end of the day, it is likely that we will say that the relationship has not been proven..
Another check is to look for common matches. If we have common substantial matches with most people who have tested on one side of my family in addition to the match with me, I am interested; if someone matches one cousin out of 5 on my father's side and one cousin out of 4 on my mother's side, I am not so interested.
Be aware that sometimes statistical probability is just plain wrong. Most of my second cousins and second cousins once removed are in the 200-400 cM range on FamilyTree DNA, with a longest match in the 20 - 40 cM range. But I also have one known second cousin once removed where the match is 79/19.
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Author: Mastering Art Law (2d ed. Carolina Academic Press 2020)