These are reminders to the pianist where the fourth finger will land.
All Diatonic Scales (Major and Minor Derivatives including Melodic, Harmonic and Natural) consist of 7 distinct notes before the octave is reached. To play these scales you will notice a reoccurring fingering pattern thus: [1-2-3]-[1-2-3-4]
For Example:
Ascending Scales - Right Hand:
C, G, D, A and B Majors all have this exact pattern.
F Major starts at group 2 - [1-2-3-4]-[1-2-3]
Eb Major starts mid-way through group 1 - ..3]-[1-2-3-4]-[1-2...
etc...
Finger Substitutions
You will notice that in your ABRSM book Eb is indicated to start with finger 2 - you can just as easily and correctly start with 3. This might be indicated thus (3) on the score?
You will further notice that at the top of the scale 5 is indicated for C, G, D, A and B Major - this is usual for ease of 'turn-a-round' before you descend the scale - but you could also quite as easily play 1 ... (Although this might not be recommended by the majority !)
Hopefully this helps to understand the quick 'reminder' at the top of the scale page. You might prefer to write your own reminder thus: RH [1-2-3]-[1-2-3-4] or simply RH [1-2-3]. LH ..1][4-3-2-1]
HINT: B Major is by far the easiest scale for the Right-hand as the notes naturally fall under the hand when practising passing your thumb under and F# Major for the left hand... CHECK YOUR BOOK FOR FINGERING and not the reoccurring pattern - I've not touched the Left-hand or descending scale patterns here!
Happy Scaling the heights of Piano Technique - Scales !