Mainly their scope of application: Structural Steel as in buildings compared with Heavy Equipment as in earthmovers, log stackers, dozers, etc. (I hope I have the correct D14 in mind when I say that)
The equipment needs more care in some areas similar to what is supplied in D1.8 for Seismic in addition to D1.1.
Also, D1.1 was originally the base code for many others that while they have been made more of a stand along code for their application still highly resemble D1.1 in many areas as welder qualification.
Brent
Your quote is from the 2005 edition of D14.3.
The most recent, or 2010 edition expands on this to say that welders qualified to AWS QC7 at an ATF, along with "...or other standards acceptable to the Manufacturer may be considered qualified within the limits of 8.2.2."
(8.2.2 is the WPQR essential variables from AWS B2.1, with a couple additions like recognizing that GMAW transfer modes, Spray , SC , pulse are separate processes when it comes to qualification.)
The upcoming 201X version will expand further to include ISO 9606-1.
The differences between D1.1 and D14.3 are that D1.1 is the base code and D14.3 is a specification.
As a spec, D14.3 includes additional design and welding requirements that affect mobile equipment and associated ROPS (Roll Over Protective Systems), while excluding most all of the building related stuff.
On a side note, I'm happy to say that D14.3 includes requirements that D1.1 is starting to leave out and which are giving it the non-flattering nickname "The New Farm Code".
Don't forget that when using D14.3, you must also use D14.4: Specification for the Design of Welded Joints in Machinery and Equipment, for design considerations.
Tim