There are now 5 possible Side codes that can be used for a sell order; Sell; Sell Plus; Sell Short; Sell Short Exempt and Sell Undisclosed. If you have a block order which contains allocations with different sell side codes, what should this value be set to on the order that is released to the broker? Seems to me there ought to be a “Mixed” value.
I think it would be better to use 2=Sell on the block level. In principle, it may be a better idea to identify the individual orders with separate messages and tie them together with ClOrdLinkID(583). Or you use 2=Sell and simply state in your Rules of Engagement that block orders may be split into allocations with different sell codes.