Here is a basic guide to what’s what in the compost world.
John Innes composts are based on loam. Loam based composts tend to have better water retaining attributes and to re wet more easily than peat based composts. Loam is a fairly heavy material and it is particularly useful in containers where the pot needs to be stable enough to copy with a tall plant or specimen plant without toppling over in high winds.
Multi-Purpose compost speaks for itself. It can be used for seeds although I prefer to use seed compost which is more suited to the growth and low nutrient requirements of seedlings. I find it very useful for potting on, planting up hanging baskets and containers and for use at the bottom of the planting hole when planting shrubs in the garden. Multi-purpose is ideal for the gardener who does not wish to buy several types of compost for particular tasks.
There is also a proliferation of compost for specific purposes such as tub and basket compost which contains a high potash feed (to encourage flowering) and water retaining gel. It also has a more open structure to help watering.
Ericaceous compost is a must for plants which hate lime such as Rhododendrons, Azaleas, Camellias, Acers and some Heathers which would struggle severely in ordinary compost.