I think the 5yos among us might still be unclear as to why/how "73" tells the 5 offensive lineman that they need to block for Brady without any additional help. What does the "7" mean? What does the "3" mean"? I was a lineman in high school in an offense that, for it's time, was relatively complicated. The vast majority of our pass plays had either "60" or "61" blocking. Despite playing in it for 4 years, I couldn't tell you why it was numbered that way. I understand the 0 and 1 referred to essentially right and left. Maybe the 6 was in reference to the number of blockers, though I don't think so. It's clearly not that way in the system referenced above.
So this is the best answer I can come up with. Hopefully someone can expand on it and/or correct it.
There are 4 basic pass blocking scheme concepts. 50's, 60's, 70's and 80's.
Note: There are different identifiers for Run Blocking schemes.
So it's not an individual designation. e.g. 73 doesn't mean 7 linemen or blockers etc.... although even numbers seem to mean right and odd numbers left.
What each Pass blocking scheme identifies:
1.
Whether Assignments vs Man or Zone and /or vs 3/4 or 4/3 Defensive fronts.
2. Who and how many are responsible for pass protection.
3. Which Defender each player is responsible for.
4. Which Offensive player is the Hot read.
5. The QB's drop. (Under center, Shotgun, 3,5,7 step drops)
6. Who the QB should identify as the Mike.
7. QB Audibles.
8. And a few other things.
So back to your (and my) question of what does 73 mean.
It means: (I think)
1. The QB is under center and I believe a 5 step drop.
2. Only the Offensive linemen are pass protecting. RB and TE are releasing into passing routes.
3. One of the Receivers/RB/TE will be a Hot route.
4. And more that I'm not sure about.
Edited. Thanks NE-VT.