Subsection stands for a section within a section.
Indeed, there are sections that consist of two subsections.
Let us see an example.
Part PI consists of two sections: section d, that is to say PI-d, and section t, namely PI-t.
Section d: look at PI-d
But section t consists of two subsections:
The first one -subsection t1- is that which has the infix put in first position of the two possible positions: between the first and the second root consonants.
Look at PI-t1
The second --subsection t2- is that which has the infix put in second position, between the second and the third root consonants.
Look at PI-t2
For both theoretical and practical reasons it is quite necessary to clearly separate and distinguish the two subsections.
Important: the Part shortening and the Subsection shortening are always united by a hyphen: PI-t1, IS-t2, PIS-t1, PIS-t2... [and not by a dot (.)]