It doesn't matter what it is, but unless it is painted blue and has that wheel added, it will be a total failure.
Which is why good consultants write good acceptance tests and make sure that while they can volunteer to paint it blue, and put a wheel on the left-hand side, they can't be forced to paint it blue, and put a wheel on the left-hand side - a change to the specifications - without the client recognizing the extra cost involved.
But think again - by asking that it be painted blue, and a wheel be put on the left-hand side, the client is agreeing that the job is almost complete. This is the consultant's cue to ask "And will this mean that the project is complete?", and get sign-off on the job.
By asking that it be painted blue, and a wheel be put on the left-hand side, the client is stating that they remain comfortable with the project, and it is indeed shaping up to be the solution they perceived three months ago.
In short, the client is still engaged in the project.
The alternative is too horrible to contemplate, let alone to write about ...