Monk fruit sweeteners use extracts containing sweet-tasting mogrosides from the monk fruit. Concentrated extract is much sweeter than ordinary sugar, so many retail products combine it with a bulk ingredient such as erythritol or allulose.
These blends are not interchangeable with one another or with pure extract. Read the package's substitution directions. Replacing sugar changes more than sweetness in baking: bulk, browning, moisture and texture may also change.
Reference: Further reading.