The volume shown on the DMEX chart and in the trade history represents an aggregation of the price source volume and internal DMEX traded volume. The last column in the trade history table shows whether the trade shown was performed on DMEX (internally - "I") or on the price source exchange (externally - "E").
When an external trade is shown in the Trade History it is generated by the one side of the order book overlapping the other side. For example, if a buy trade shows up in the trade list, it means that the buy side of the order book overlapped the sell side of the order book. This is important because it means that if your order was at the bottom of the sell side and the buy side overlapped it, your order would have been filled.
In other words, although not all trades shown in the Trade History and the chart are trades performed on DMEX, if your order would match the price of the trade, the order would have been filled. Therefore, for trading purposes the volume shown on DMEX is indicative of what is happening on the price source exchange and is effective in the sense that the volume is filling orders and triggering stop-losses.