What is the meaning of this datasheet diameter notation?

It means that there are 5 (quantity) of these holes, each has a diameter of 1.2 mm (+0.1 mm / -0 mm).

There are 5 holes (marked in blue) that need to be drilled 1.2 mm diameter.

It doesn't mean 5 minus 1.2 OR 5 minus \$\phi\$

It means there are five of that hole somewhere on the diagram. Each one is nominally 1.2mm in diameter and can be up to 0.1mm larger than nominal or 0mm smaller than nominal.