This is related to parasitics extraction in spef files.

All I know is that In IEEE standard spef files, there could be asymmetry
between the coupled nets. Consider this case:

D_NET foo <value>
1 foo:1 bar:2 <value>

Here, there is coupled capacitor between foo and bar at nodes 1 and 2 respectively with value of <value>.

What I do not know is that 'What exactly people mean when they say that there is asymmetry between the
coupled nets?'

I tried to google about it but could not get an clear answer.

Does anyone here know 2 cents about it? An example would be great.