I found it east to define my areas of focus because i complete a life chart every 3 months and that means i think about my areas of focus:Career,Family, Finance, Myself, Health, Relationships & Community. This last one i added after encouragement from this community. Most of my projects fit nicely into one AOF all though others such as finance may come into play. When i do a life chart and think about my AOFs, i create a document which tells me what i think of each area and how satisfied an dissatisfied i am. I may decide i want to do something about one area of focus. for example, if i'm unhappy about my career, i may decide to change jobs or just do some training at work or home, such as a new course or read a book. At this point i will try to come up with a new project(s), goals or vision. Eventually my ideas in these 3 horizons will solidify, sometimes there wont be any projects immediately , it may just be a goal to pick up new skills for a career change, the up shot is that i will come to a conclusion that may involve a new visions, one or more goals and/or one or more projects. i may even tone down my ideas just to one course (at the project level). This is the main way i use my AOFs.
Another thing i do is to classify which AOF my projects belong to (if possible). if i have 6 projects for my computer, i'll only want to do one at a time so as not not mess my machine up. So on my list of projects i will filter on AOF (Computer comes under Self as its a hobby) and decide which i want to do.
As i have each project in at least one AOf , it tells me how much i am focussing on each AOF. For example , i have lots of projects on self and career , but none on health and relationships. These are areas i want to improve thanks to my life chart and use of my AOFs.
Hope this is of use.... good luck