Trying to decipher the hole or shaft sizes in a stepper motor diagram
The dimensions give the hole diameter, and the outer radius of the tabs.

The position of the arrows indicate where things are being measured from. In the case of the radius and diameter measurements, these are from the hole centres as indicated by the lines extended beyond the arrow heads into the centre of the hole.



Lets start with the hole diameters

2 - ɸ4.2±0.15


Broken down this gives:

2     - The dimension applies to two holes in the diagram - both tabs in this case
ɸ4.2  - The hole diameter is 4.2 units (probably millimeters)
±0.15 - The holes have a tolerance of plus or minus 0.15 units




The tabs are then described in terms of an outer radius to give you an idea of the size of the tab and how much material there is, and in this case by extension, how big of a screw head or washer can be.

2 - R3.5


Again broken down:

2    - Again the dimension applies to two holes, both tabs.
R3.5 - The tabs have a nominal outer radius of 3.5 units




The measurements hint at the holes being designed for M4 clearance as noted by @Brian in the comments. 

However if planning to use an M4 bolt, the maximum head diameter of 7mm indicated by the tab sizes would require use of either a small-head hex bolt, or at a push a pan head bolt. A standard M4 bolt head diameter may well be too large.