In many cases, this type of issue will occur when the offer is subject to specific terms on the allowable quantity. For example, if you hold a quantity of 5,000 and the terms of the offer indicate that "for every 3 units you may purchase 1 new unit", your participation will be limited to a multiple of 3.
In the event an account will hold additional shares after an election, for instance in the above example the account could submit an election for 4,998 shares and will be left with 2 shares, clients should review the terms of the offer to understand what will happen with un-elected positions. In the case of a subscription rights offering, you may be able to sell the remaining shares if the rights are listed on an exchange.