Quotation rules are built on round lots. A protected quote under reg-nms has to be for at least a round lot, and depth shown in level-2 is expressed in lots at many venues.
Exchanges have moved to variable round lots based on share price so that expensive stocks are not effectively unquotable, with smaller lot sizes for higher-priced names.
Example: in a $10 stock the round lot is 100 shares, a $1,000 order. In a $2,500 stock a 100-share lot would be $250,000, so a lot size of 1 or 10 is used instead. Anything below the applicable lot is an odd-lot-order.
Related: odd-lot-order