USDT Dominance measures USDT market cap against all other coins. While not as potent as BTCD or TOTAL2, it can be a helpful indicator to know what traders are doing, as many of them use USDT over fiat.
Obviously, when BTC sells off, USDTD spikes. Based on a little TA, we might expect a relief rally for the market soon. USDT is basically at resistance, but it did put in a slightly higher high. Recent volume is decreasing, signaling a reduction in selling pressure. RSI is nearly at Overbought levels (which would be considered oversold for the rest of the market)
Just remember - When you think BTC can't go any lower... it always can.