The specification IMI318 refers to composition rather than a specific microstructure. These Ti-6-4 alloys can be produced with a very wide range of microstructures by using various thermomechanical treatments. See for example this book, fig 3.11.
I could speculate that the A refers to the annealed condition. Suffixes to the IMI318 designation are common, such as MA for mill annealed (from the alpha+beta phase field) and BA for beta phase field annealed. The first answer correctly states that this alloy can have a huge range of microstructures. Another good reference is Ian Polmear, "Metallurgy of the Light Alloys". The 3rd edition can be bought second hand for just a penny (plus P&P) from Amazon UK....