Conversion naming

[9DSX_DRVR]

Alright, I hope I ask this question the right way, so here goes.

There are a lot of conversions for the 180sx/240sx were the front end is switch out for a different version or body.

For example:
Silvia front end + 180sx body = Sil-80 or Sil-eighty
180sx front end + Silvia rear end = One-via or 1-via.

From this example, it seems that what is in front of the car is the 1st part of the name and what is in back or main body is the 2nd part of the name.

So, here is my confusion.

For example:
s14 or s15 front end + s13 back end = s14.3 (according to above)
but i have seen instances where someone would call them s13.4 or s13.5

I have seen examples of:
s13 front end + s14 rear = s13.4 and the next person calls it s14.3

so my question is:

Which number comes 1st, the front of the car
or the main part/rear of the car.