If you're talking about money management, and what constitutes a "unit" for betting purposes, here is how a lot of people do it ... assuming a unit is, say, $100 :
1. If you're betting on an underdog, then you bet to WIN a unit .. so if you're betting on a +200 underdog, you're going to risk $50 to win $100.
2. If you're betting on a favourite, you RISK a unit ... so if you're betting on a -300 favourite, you're going to risk $100 to win $33.33.
This technique helps to adjust for the relative risks of your bets, and the probabability of each bet winning.
And as your bankroll changes, your unit sizw will change with it. If your bankroll is $10,000, then yout unit size of $100 is 1% of your bankroll. If you're sucessful, and you increase your bankroll to, say, $12,000, then your unit size should be $120 (1% of $12,000).
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In addition, as Royalfan already mentioned, a lot of successful bettors are able to judge the "strength" of each bet that they make, and if they deem a particular bet to be very strong, they will bet more than 1 unit on that particular bet (they may have a ranking system, based on the strength of each bet, and may vary their unit size between, say, 1 and 5 units based on the strength of the particular bet).