There are several odd lot settings to be aware of for post trade allocations. By default, the setting to Allocate in Round Lots only is active.
When this is checked, allocation percents are applied to an order as defined, and the calculations are then rounded up or down to the nearest round lot (i.e. for stocks, to the closest 100 shares). The remaining shares are then allocated to the account designated for odd lots (specified by the Odd Lots radio button on the allocations page).
For example, if you allocate a 1000 share trade across three accounts, A, B and C, with percentages of 33%, 33%, and 34% respectively, and account A is designated for odd lots, the allocations shall be calculated as follows: A = 400 shares, B = 300 shares, and C = 300 shares.
When the check box is not selected, allocation percents are applied to an order as defined, but the calculations are not rounded to the nearest round lot. However, you must still designate an odd lots account that is used when the quantity does not divide evenly among accounts. The remaining shares are allocated to this account.
Using the same premise as the above example, in this case the allocations would be calculated as: A = 330 shares, B = 330 shares, and C = 340 shares.
In another example, if you allocated 349 shares evenly across three accounts (116.33 shares each), the calculations are each rounded down to 116, and 116 shares are allocated to two accounts and 117 shares are allocated to the odd lots account.