Here's the deal.
The 4/3 stores the appellate record -- briefs, reporter's transcripts, clerk's transcripts, miscellaneous filed documents, and appendices -- in plastic magazine file boxes. The file boxes are about 11 1/2 " tall, 4" deep . . . and 8 3/4" wide.
But very wide side tabs are discouraged. If the tabs stick out more than 1/4" and won't flex a little, the appendix wont' fit into the file box.
And bottom tabs are forbidded. Appendices with bottom tabs won't sit flush in the file boxes. They'll just wobble around until the bottom tabs are crushed. That's no good.
Top tabs? I haven't seen any, but they would fit, I guess.
TRIVIA: Around here we call the file boxes "doghouses." A case with a short record, less than 4" thick, will be a "one doghouse case." A case with a lengthy record may be a "five doghouse case" or a "10 doghouse case" or, as I've seen, a "35 doghouse case." I've heard tale of 100 doghouse cases . . . .