V sets were originally designated U sets, however when they were fitted with vaccum retention toilets they were recoded V sets.
lol I had a good chuckle there. Seems random, but logical.
But let's remember, while we're talking about target's, M sets were 4 car Red rattlers consisting of 2 single deck motor cars and 2 double deck trailers, H sets were 4 car single deck sets and T sets were 2 car Double Deck sets (2 car L sets really).
It seems like they are trying to reuse former target plate designations too? 
Well, here's a small history of the target plates.
In 1926, the H target was brought out for use of the Single deck sets. I believe the F set target was also in use. The H and F targets were similar.
Following that, the S target plate was brought out for use of the newer production of single deck cars of Comeng in 1955. These cars had fully automatic doors and 120V.
In 1956, new Single Deck Intercity rollingstock was brought out and were targetted "U" Sets. This probably meant "Inter-Urban" sets.
In 1964, following the arrival of 120 new Double Deck trailers, "M" targets were brought out for new 4 car sets; 2 single deck motors and 2 double deck trailers. Following this, trailers T4701-4740 were converted and renumbered T4751-4790.
The first Double Deck set in 1968 carried target "S10".
In 1970, new Intercity cars were brought out and were targetted "U" sets.
In 1977, new Intercity cars were introduced and were fitted with Vacuum retention toilets, which gave them the "V" target. For there Vacuum retention toilets. The V target is use and was used for the next 4 batches of DD Intercity rollingstock.
In 1972, when the first of the Comeng Double Deck Suburban cars were introduced, the S sets were given the "W" set target, so the S target can be used for the new sets. 2 car sets were given the T target, probably for "Two" car sets.
Between 1981-1985, R sets were introduced. These cars were the first "Air-Conditioned Suburban" train introduced in Sydney. The series II cars were anyway. And in 1987, new K sets were introduced as refinements of the previously introduced R sets. The K sets were the fitted with the chopper equipment that was also fitted in the latter V sets and Tangara's.
When the Tangara was introduced in 1988, a whole renovation was made to the Double Deck fleet targets. K sets were given the "C" target for chopper control, R sets were given the "K" target because the "R" target was going to be used for 6 car sets. S sets kept there target and 2 car T sets were given the "L" set target, so the new Tangara's can use the "T" target.
By 1989, some of the H and F sets had been withdrawn and scrapped due to the introduction of new Double Deck trains. The "F" target plate had almost disappeared after the mid 80's. But one set that consists of 3426, 4673, 4527 3218 and sometimes 3653 and 3102 have the "F1" target plate.
The "G" target was new. Also, the first zoo train carried the "H22" target in 1991.
In 1994, following the G set introduction, the Single Deck U sets were withdrawn, ending the "U" target. This target hasn't been used since.
By 2001, almost 10 years since it disappeared, the M target was back and used on the new Millennium Trains. And it's clear that "M" stands for "Millenniums".
And in 2006, the "H" target had re appeared after over 10 years of absence. It was used on the now running OSCAR units. I don't know at this moment what "H" stands for, but it's used.
Hope this helps.
