Factories Act is an Act which provides for the health, welfare and security of the workers employed in a factory. It does not speak anything about the employee skill levels, wages or service conditions. The only Act which categorises employees into unskilled, semi skilled, skilled and highly skilled is the Minimum Wages Act and that also does not speak how to promote an employee from unskilled to semi skilled or from semi skilled to skilled. That means it depends on the employer to decide whether one is unskilled or skilled. It normally depends on the education/training obtained and the level of supervision required for a worker to get the work done. As such if a worker has no formal education or training in the particular work he can be put under unskilled. One who has obtained basic training but cannot work without any direct supervision he shall be put under semi skilled. One who has obtained know how to do a work independently without a supervisor guiding him can be put under Skilled. The cream of skilled workers who can even manage the show in the absence of supervisor will be put under highly skilled worker.
If, in order to reduce the burden of payment of wages, an employer keeps an employee under unskilled, it can be questioned by the worker concerned but the decision to upgrade his wages depends on the employer's decision. The worker has to establish the above parameters so as to get an upgradation. In a Unionised environment it will be easy to but in other circumstances, the employer will keep the worker under semi skilled so long as he is not going to leave the organisation. Naturally, when he shows the sign that he is employable and will get an opportunity as a skilled worker some where else, the employer will offer a promotion or grade change to skilled worker. This is how the categorisation works in general.