I have been primarily a set collector (I'm sure I've mentioned this before.) One of the principal aspects of putting together a set is collating the set. And how does one collate a set released by the card manufacturers? You collate them by number.
This is to say that each part of the set had an identity and the main way to distinguish a card's place in the collation was through the card number.
Embellishments of the card number also occur. When Topps began its Topps Traded run, the numbers were suffixed with a T. Score also suffixed theirs with a T and Fleer prefixed theirs with a U (for Update). This practice is still continued in certain sets to this day (witness: Topps Update and Highlights). These were only distinguishing numbers. Many insert sets continued this practice and still do presently.
The practice of numbering, then, is quite ubiquitous for all manner of sets. There are exceptions, though. One example (if I'm not mistaken) is the 2003 Bowman's Best set. The cards are numbered BB-player initials. This makes it feel as if it's not a set and instead, is a random collection. How would one collate this? By first initial? last name? ones without and without a middle initial? team? To me, it disrupts the natural order of collation.
Another common occurrence is with Relic and Auto sets; these are more understandable because, to an extent, the cards are more stand-alone. However, the initial don't always make sense either. RO for Rodriguez? or They are not a product unto themselves and are inserts.
However, even with insert sets, the trend is tiresome. My least favorite example is the 2009 Topps Legends of the Game insert set. The first series is numbered LG1-LG25, the second series is numbered by player initials or something, and the updates and highlights series is number LGU1-LGU25. Where's the continuity for sets with the same design continuity would allow for completeness. Topps Heritage High Numbers is a continuation of the first series in concept and numbering. I have no idea what Bowman Draft Picks and Prospects continues except schizophrenic order.
What are your thoughts on collation and numbering? Even if you're not a set collector, what identifiers do you look for in a card?