It's some sort of internal EVA naming. What I hold as significant about it is that it is not anything I have ever seen in the past from EVA.

Go to http://www.evaair.com
Go down to the "TIME TABLE". Middle bottom of the first page.
Put in
From: Taipei
To: Los Angeles
one way
Departure Date: Mo 5 Day 29
and click INQUIRY.

Notice BR12 is a 74W four days of the week, a B747-400 one day and a B777-300 two days of the week. Seemingly indicating 3 aircraft types.

Now click on "PREVIOUS WEEK"
Notice BR12 is either a B747-400 or a B777-300 depending on the day. No "74W" that week.

If you check again on
Departure Date: Mo 8 Day 18
You see BR12 still has 74W two days of the week. Hit NEXT week and BR12 is all B777-300's, the 74W is gone.

So where does the mystery plane go?

Try
From: Taipei
To: Vancouver
one way
Departure Date: Mo 8 Day 17
Notice BR10 is a B747-400 3 days a week
Click NEXT WEEK
Notice BR10 switched from a B747-400 to a 74W those days. Immediately after LAX switches to the B777-300. Thus probably it is the plane LAX is giving up.

What does all of this mean? It means, I spend think way to much time thinking about this stuff but really don't know what the heck it means at all.

Just that I can see.

74W (????????)

B747-4E (744 combi)

B747-400 (in the past used to indicate all configurations of 744's except combi's)

B777-300 (777-300ER)

And in the past EVA used 77W when the new 773's were introduced.