Why Variant Tagnames?
1 Many control system support them, often they are named Aliases. They are used mostly to make one graphic that covers several similar equipment entities. ControlDraw variants are the best way known to S88 and science of making the lookup tables that relate the Alias tags to the real ones. You can define alternative tagnames and immediately see them on the diagram (or call it the graphic design)
2 The split of a plant into P&ID's does not often match the S88 physical model.
For example, most projects have a P&ID for each 'process entity' - if you are lucky it will nearly be a unit, but often it will be part of a unit or a few parts of several units. When it comes to applying tagnames to the instruments on the PID the Instrument engineer will try to use parallel numbering so that the instruments in the 'streams' are all similarly numbered. However this does not cascade into the S88 object numbering if the modularisation for S88 is not the same as that for the P&ID's.
3 In the ideal world no mistakes are made.
In the simpler world of instrument tag naming mistakes are often made. This is one more reason why ControlDraw includes Variant Tagnames.
Given 4 P&ID's each with 50 tagnames, and one hour, there are 200 tagnames to be assigned. That is 18 seconds for each. Mistakes happen, even (especially :-) ) with a computer instead of a pen and paper. In that 18 seconds you have to which decide pattern the number should follow, find the corresponding value and then commit it. Over the course of such 200 decisions and manual actions mistakes will happen, maybe just in the typing. It can take long time before such typo's are evident. It may be cheaper to leave the 'hard' tags as they are and use ControlDraw Variant tags.