Dear Jiwanjot,
It is not correct that the word 'contractor' has not been defined in the BOCW Act. It has been defined in section 2(g) of the BOCW Act. The word 'Principal Employer,' although not defined in the Act, finds a place in Form - I of the BOCW Rules, 1996; as you are aware, the rules are made to achieve the object of the Act. This reveals the intention of the parliament as to who will make an application for Registration of the Establishment. Punjab has defined the word 'Principal Employer' in its BOCW Rules, 2008 in clause (zr) of Rule 2.
Also, please be clarified that in BOCW, it is the 'establishment' which is required to be registered and not the employer. The registrable establishment is where the building and construction work is carried on. In this connection, kindly read section 1(4), which tells us what establishments the BOCW Act is applicable to. Keeping this in view, then read the definition of 'establishment,' then of 'employer,' and then section 7 of the Act, which tells us who is required to get the establishment under construction registered under the Act.
Now the question would be who is the 'employer' who is required to make an application for registration of the establishment which is under construction. For this, kindly read the scheme of the Cess Act, which further clarifies the 'employer' is a person who owns the establishment being constructed and who incurs the cost of construction for the same. He is the person responsible for payment of Cess, and for this reason, for private construction, the owner is the employer; at the time of approval of building plans, he pays the cess; there are no contractors involved at that time. Since for private establishments, the owner is the employer, therefore, he has to make an application for the registration of the establishment which is under construction. However, in the case of the construction of any establishment of Govt. or PSUs, the contractor is the employer as defined in sub-clause (i) of section 2(1)(i) of the BOCW Act, 1996.
Best wishes,
Jpratap