Just to clear up some of the confusion here:

-It's magenta because it's been confirmed.

-The offset track is dashed, while the main route remains solid

-It is a feature on the 777X (as it was on the NGX)

 

Here I am testing it out to grab a screenshot for someone who requested it (that's a hint for some of you):

 

Note the XTRK value on the offset (and you can see the format and location of the entry on the FMC, pictured).

 

 

 

...and not to say that it cannot be used for weather, I'd honestly just jump around weather with HDG SEL (might be a habit because I don't have LNAV is the planes I fly).  I've really only used offsets to sidestep traffic, and SLOP on the NATs.

Offsets can be useful to visualize on the ND where your ATC clearance for WX avoidance ends.

Offsets can be useful to visualize on the ND where your ATC clearance for WX avoidance ends.

 

I guess that's why I added that last piece in my last post.  I was never taught to to limit it to X many nm left or right of course.  It was always degrees.  Granted, you could give a nm restriction like that.  I'm not trying to say you can't.  It's just that I'm used to:

"Deviation up to 30 degrees left of course approved, advise when able direct [point]."

 

Heck, when I was flying CKM-BCB, I just got:

"Deviations for weather approved, advise when able direct PulaSKi."

 

(This is one of the reasons I always get on people for assuming IFR affords pilots zero discretion in terms of horizontal path.  This is one example of many that do.)

Under radar coverage, you normally won't get a limit for your deviation (controller can monitor your progress and separate based on that).

 

However, under non-radar RNP airspace, the controller will almost always use a lateral limit to the deviation (e.g. deviate up to 30 miles left and right of track). This is because the controller must still achieve standard separation while you are deviating from track. Sometimes it is not possible to grant a sufficient deviation while still maintaining separation; interestingly, in this situation, the controller must advise "unable due traffic; advise intentions" and at this point, the onus is on the PIC to declare an emergency to deviate around the weather (perfectly acceptable and normal practice) and the controller issues traffic information.