The major difference it makes is if you want your 3 phase motors to rotate in the proper direction, if you want to synchronize your generators with the grid, if you want your transfer switching to take place without incident, etc., etc.

anti-clock wise

Which ever way you want it to be.

Just look at the NDE instead DE of the motor.

It makes no odds anyway, just swap two phases.

RYB in temporal order.

The main thing is to be consistent from place to place within a facility.

Now you've hit the nail on the head.

Serving my time the works had originally it's own power station, it ran reverse to the grid. We were eventually connected to the grid. All the old plants were at 550V, and then we had new plants built running on 433V. Some idiot decided that the new plants should have the "correct" phase rotation. Chaos ensued!

As engineer on a major plant alteration that would merge new in to old I went for "we'll go backwards"! I even had to have switchgear painted different colours because there could be two different voltages next to each other. Red for 550V, Blue 433V.

and which standard are you talking about?

Well, the Standard of Phase Rotation (Phase Sequence) is usually set by the Electricity Board in any Country for example some one who knows can tell us how it is in UK as declared by the Central Electricity Generation Board (CEGB)

Well, it goes the other way in Liverpool to many other places, according to urban legend...

The main reason is standardization. Three phase SCR controls are phase dependent and will not work without correct phasing. Of coarse three phase motors are reversed by changing any two wires. Sometimes transformer are used three phase in and then are used as single phase out. You must know how the input phrasing is arranged to prevent overloading a single phase.

what differance does it make?

Connecting two three phase systems with different phase sequences is counter-productive (for obvious reasons).