Essential oil producers have found less expensive ways of producing certain essential oils, which would otherwise have a very high price tag. They can produce what is called a standardised essential oil, meaning a standard scent is produced by blending various species of the plant. Multiple species of some plants are more cost effective and easier to grow than some specific individual species. For example the rose blend essential oil is a blend of rose species: its INCI is rosa spp (spp is botanical shorthand for multiple species), as opposed to rosa damascena (which is a very costly rose species and the essential oil is eye-wateringly expensive).
Standardised essential oils have a standard scent, whereas a single species essential oil may vary in scent because it is an individual plant species that can naturally vary with season, rainfall, soil quality, geography and a variety of other factors. Standardising the scent by combining multiple species is one way of getting a product that will always be similar in quality. Standardised oils can therefore be quite beneficial when scent reliability is required.Standardised essential oils are still pure essential oils, and have not been diluted, filled or altered in any way. The only differentiating factor is plant species used.