Yes "adjacent" is relevant in so far as calculating the aggregate 10 litre capacity.
If the kitchen is fitted with two 5 littre fryers which are at opposite ends of the kitchen, then they are not aggregated to 10 litres which would require a fixed extinguishing system.
I believe that the authors of FPN4 were of the opinion that if the capacity of one fryer, or a number of fryers that would potentially be involved in the early stages of the same fire, was greater than 10 litres the risk was significant enough to warrant fixed extinguishing, if the capacity was less than 10 litres, the risk was considerably smaller.
I don't think that the 10 litres criteria is much more than an arbitary figure, I am not aware of any research carried out by DH which demonstrates that anything significant in fire terms occurs at the transition between 9 litres of oil or fat, and 11 litres of oil or fat.