What’s the difference between “approved” and “legal” procedure? I occasionally see these two words used in opposition; for instance [emphasis mine]:


  In the United States, aircraft usually join the pattern at a 45° angle to the downwind leg and abeam midfield. Although aircraft may legally join the pattern at any point, the AIM clearly states that the only approved pattern entry is the 45[° angle entry].1


In the referenced circular AC 90-66B CHG 1, the words “approved” and “legal” don’t appear except in other contexts.

So,


Is this just imprecise layman terminology in use on Wikipedia?
Or do the two terms have defined meaning somewhere?


My guess is that a pilot-in-command can deviate from “approved” procedure to other “legal” procedure whenever deemed necessary by exercising PIC authority, but deviation from “legal” procedure would be allowable only in emergency (“mayday”) or urgency/distress (“pan-pan”) situations? Or perhaps also if controllers direct or authorize deviation?2

I’m curious primarily about flying under FAA and ICAO regulations, but if this terminology differs in other jurisdictions, I’d be interested in that as well.


1 From English Wikipedia, “Aircraft traffic pattern”, retrieved 16 Aug 2019 20:30Z 

2 Wording in the circular §8.2.1:


  The FAA does not regulate traffic pattern entry, only traffic pattern flow.


gives a clue in specifying that the 45° entry is not regulated.
The AIM is not regulatory, just best practices approved by the FAA. You should do what the AIM says, but as PIC you are free to ignore it.

The FARs are regulatory, so as long as you don't violate them, what you're doing is legal. You must do what the FARs say (unless you have an emergency, of course).

In the case of pattern entry, other pilots are expecting you to do what the AIM says (45 to downwind), so doing something different is probably unsafe--especially if you aren't in radio contact. But if there is nobody else in the pattern, that reason doesn't apply and it is safe to enter however you wish (straight in, crosswind, etc.). It is up to you as PIC to make this decision.

The same logic is true generally: if you understand why the AIM says to do (or not do) something, you probably understand when it's okay to ignore it.