Process are the things that we have students do that we think will result in them obtaining the level of expected level proficiency. This includes things like participation, work habits and the big one.... homework. If you want to get a little more specific you could even say attempting homework. Are all of these things important? Of course they are, but they are not the same as product. Can a student do none of the homework and stll pass all the tests? Yes. Is it unlikely? Probably.
If a player on a sports team or student in a band doesn't practice, will their performance be sharp? Probably not as good as it could be, but it might be (see Allen Iverson example).
Process is important (punctuality, participation, effort) but it's not the same thing as achievement. You can still collect process grades in SBG, but they shouldn't be used in calculating the academic grade. So what's the solution? You can put multiple grades on your report cards (and transcripts), but once you start combining them you introduce noise to the signal. Below is an example of a standards based report card that seperates Academic Achievement (product) from learning behaviours and attitudes (process). Both are important, so both go on the report card.