Buttons are straight forward, Start, Clear, Stop. With nothing in the pane, it's already cleared. To pick something for download, one sets up a search criteria (below) then clicks on one or more assets to select them (Highlighted, colors of CM and highlighting are as you set them in your Windows theme) then drag them into the DLH pane. As you add more you'll likely note some activity going on, part of which is CM and the DLS are filling in dependencies—sub-assets the asset you selected depend upon as parts, for example the bogeys, wheels, or couplings of that railcar type, or textures, etc. which are parts of the assets you selected. As you add more by drag and drop, the list will grow a scroll bar and grow longer as dependencies are identified and added.
You should always give the DLH some time to settle before hitting Start, but once it's cranked the top of your list for a couple of minutes, these days hitting start seems risk free, unlike some of the instabilities CMP used to exhibit as are alluded to in the CMP article. CM-2.0 may suffer some of he same issues, but CM-3.3 and after offload much of the data management to TADdaemon.exe which gates data to and from the DLS whilst TrainzUtil.exe manages adds, extractions, and so forth to the database proper; both being invisible in the background save to power users.
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