As time goes by, the subsequent splitting of blocks that proof-of-stake generates will reach the province of tiny amount of HYP, to the point they go below transaction fee - in other word, not enough coin to even send the said coins, rendering them unusable.
To prevent this, presstab implemented a threshold at 166 HYP. When splitting a block for staking would end up with one of the block being less than 166 HYP, the wallet will combine the said block with another one, until the resulting block is above the 166 HYP threshold.
There is a threshold where the "dust" should be collected. This is an area I need to do more research on, and haven't seen happen in my own experience. The code is written so that theoretically dust should be combined when blocks are less than 166 HYP. This dust collection happens when a coin stake is created. I am not sure exactly how the mechanics works… is it from when the less than 166 HYP block stakes, or is it triggered by any block staking? How I read the code at first glance is that when any block stakes and the "input" or total block amount is going to be less than 166, the wallet will grab other mature blocks and stake them in the same block. It will sum up as many small blocks as it can until 166 HYP is reached. I could be wrong about this, but this is my initial reaction to seeing the code. You can see it in wallet.cpp under createcoinstake.