[PROJ] Meaning of (x) numbers in proj transformations in QGIS

[Richard Duivenvoorde]

Hi List,

coming from this QGIS Dev thread:

https://lists.osgeo.org/pipermail/qgis-developer/2019-June/057806.html

I was wondering which proper information an average QGIS user can use to
determine what transformation to pick...

And what information (numbers and text) in these dialogs come from proj
or from QGIS.

In the above thread I point to the images in this post:

http://www.qgis.nl/2019/06/27/english-proj-select-datum-transformations-for-epsg28992/?lang=en

First screenie being a 'choose transformation' dialog in QGIS 3.8 (with
proj5), the second one from QGIS master with proj6.

According to Nyall, he uses the info from proj to label the 'best fit'
green, but I also see 'higher order' numbers in the descriptions of the
options (numbers between brackets). My main reason to come here:
apparently (3) is by proj/Nyall seen as current, but then I wonder why
there is a (4) option...

But my main question: is "which proper information an average QGIS user
can use to determine what transformation to pick.." if presented with
one of those dialogs (as I had....)... and not being a projection guru :-)

Regards,

Richard Duivenvoorde