Two questions: (1) Why maximum? (2) Can you clear this up: even though it's called "single point," with the box, it's really a group of points. Is that right?
I over generalized when I used the word "always". The AF system has many modes and many options to optimize the AF algorithms for the task at hand.
In general the largest focus region in single-point mode increases the odds the AF algorithms will succeed.
A large focus region increases the odds simply because there is more information. The AF algorithms choose the single point closest to the camera to avoid focusing on an unintended region behind the intended region.
However for critical focus in cases where very shallow DOF is desired carefully adjusting the scene position of a smaller focus region could be useful. In this case we don't need more information we just need the most important information (focus object selection) to be accurate.
It turns out eye detection is automated. So, for portraits a small focus region does not necessarily offer an advantage.
As far as I can tell single point means the AF algorithms choose a single AF point from a group of points in the focus region to set focus.
From the X100F manual:
SINGLE POINT
"The camera focuses on the subject in the selected focus point. Use for pin-point focus on a selected subject. The number of focus points available can be selected using AF/MF SETTING > NUMBER OF THE FOCUS POINTS. Use for pin-point focus on a selected subject."
"Choose the number of focus points available for focus-point selection in manual focus mode or when SINGLE POINT is selected for AF MODE.
91 POINTS (7×13)
Choose from 91 focus points arranged in a 7- by 13-point grid.
325 POINTS (13×25)
Choose from 325 focus points arranged in a 13- by 25-point grid."
Note these are the maximum number of point, not the points in the user selected focus region.
In single-point mode one can increase the size of the focus frame
"AF-S + Single Point
For capturing subjects using a specific AF point
A basic AF mode. Set focus using one of the focusing points*1 - ideal for focusing precisely on a subject in a fixed location. The focus area can be one of five different sizes and the location of the focus area can also be changed."
"The Focus Area selection screen displays all points in the Single Point mode or 77 points in the Zone and Wide/Tracking modes. In the 77-point view, Intelligent Hybrid AF using the high-speed phase detection AF system is activated for the zones at the center larger rectangles, where the phase detection pixels are embedded. The position of the focus area, the size of the AF frame and the starting point of focus tracking in AF-C can be adjusted in applicable AF modes."
"The Single Point AF area is divided into smaller sections to accurately determine the distance to the subject for even greater focusing precision. When the selected section contains both subject and background, the AF system focuses on the subject nearer the camera, to prevent accidentally focusing on the background."
"AF-S + Single Point (focus area selected)
Choose any one of focus areas[1], and change to the best one for the subject's position. The focus area can be changed to one of five different sizes, according to the size of the subject."
Spot-focusing on subjects moving backwards and forwards in a straight line
This spot-focus AF-C mode is capable of focusing on a specific area, enabling you to choose both location and size of the point within the focus area
1. The types of scenes where this mode may be used is limited, but it is ideal when pin-point focusing is required."
and
For manual focus mode (M switch on the camera body is selected) -"Focus-area selection can also be used to choose the focus point for manual focus and focus zoom."
