The "target allocation" allows you to allocate a percentage of the funds in your wallet to the trade engine for each market that you have selected.
Any current allocation is indicated in green and orange. A green bar indicates the allocation of funds (the quote currency) in open buy orders. The orange indicates the worth of funds that were acquired earlier and are waiting to be sold.
Example
Suppose you have selected BNB/BTC on Binance and had set the target allocation to 35% (for example yesterday). Also suppose that the allocation in open buy orders (green) is currently 25% and the allocation in open sell orders (orange) is 10%.
In this case, the 25% in open buy orders (green) exists as BTC in your wallet while the other 10% (orange) exists in your wallet as BNB and was acquired earlier by the trade engine.
Now suppose that you change the target allocation from 35% to 25% by moving the slider to the left. The trade engine will now proceed to cancel open buy orders in order to work towards the new desired allocation of 25%.
Now suppose you move the slider even further to the left to 5%, into the orange. To reach the target allocation, the trade engine has to cancel all buy orders. But this reduces the allocation to only 10%. The only way to further reduce the allocation from 10% to the desired 5% is to sell off the BNB. Although the trade engine will make every effort to do exactly this, it will depend on the market (and your settings, like the minimum trade profit percentage) whether the trade engine will manage.
Notes
To determine the worth of the funds in "orange", a recent conversion rate was used, not the rate at which they were bought.
Funds that are waiting to be sold via manually added "historic" buy orders or via the "Surplus selling" feature are not included (in the "orange").
It is possible to "remove" the orange forcibly by purging the current buy order history. This will make the trade engine "forget" about the funds (BNB in the example above). Check out this article for how to do that.
TL;DR
Keep selling but no more buying for 1 market: Set selected market to 0% allocation
Stop selling and buying for 1 market: Disable the selected market
Stop selling and buying for an exchange: Disable the trade engine for the exchange on the dashboard