What is the name for the beer trait which is based on the relationship between IBUs and malt content?

The correct term you are looking for that describes the beer trait based on the relationship between International Bitterness Units (IBUs) and malt content is "Perceived Bitterness." This concept refers to how bitterness is experienced by the drinker, which can be influenced by the sweetness and other characteristics of the malt in the beer.

A high IBU count does indicate greater bitterness, but if a beer has significant malt sweetness, it may balance the perceived bitterness, making it taste less bitter than the IBU number would suggest. This perception is critical for brewers, as it influences the overall flavor profile and drinkability of the beer.

Understanding perceived bitterness helps in crafting beers that have a balanced flavor profile, where the bitterness is neither overwhelming nor too subdued based on the corresponding malt backbone. It’s an important concept for evaluating and discussing the overall taste and balance of different beer styles.