The trade id of the block trade. This is used by each one of the allocated trades to reference the block trade. This element can also represent the trade id of the parent trade for N-level allocations. In the case, this element is only used to model N-level allocations in which the trade acts as block and allocated trade at the same time. This basically means the ability to allocate a block trade to multiple allocation trades, and then allocate these in turn to other allocation trades (and so on if desired). For new implementations, the use of the "linkedTrade" element is preferred.