This setting is used in combination with max_trading_pairs.
For example when you are already at your max of 2 pairs, the bot can make an exception for the mentioned pairs (ETH and BTC in the example below).
max_trading_pairs = 2
max_trading_pairs_ignore = ETH, BTC
DCA_max_trading_pairs = 2
DCA_max_trading_pairs_ignore = ETH, BTC