To what extent you believe the employment contract is able to regulate the employment relationship?

the contract should be bounded with national & international labor law.

Absolutely.Employment contract is an agreement written or oral express or implied agreement between employer and worker for a specific term or indefinite or to accomplish a particular job is committed whereby the worker to perform work for the benefit of the employer and under his direction and supervision, and adheres to the employer to pay remuneration agreed to the worker.

Employment Contracts are just a legal binding to cover a broad spectrum only.
And those days are over when you could regulate or manage your employees through contract letter.
You can manage your employees with clear understanding of goals, open communication and right guidance, and there is no other way than that.

employment contract can manage only the legal part yet will never be able to manage the psychological contract which is udisclosed terms and conditions; like the willing of the employer to work for extended hours as way to show your hard work, or his not willing to let you go for leaves and consider it uncommitment from the employee.

Aside from standard legal contractual terms there are hygiene factors also which determines the employment relationship between the employer and employee.
Please be appraised that the ability lies in extracting the work rather than simply going by pure carrot and stick method.

each agreement is a fixed term or indefinite, concluded between the employer and the employee and the employee undertakes that works in the service of the employer and under his administration or supervision remunerated by the employer undertakes ...It also includes a grant given to the employee penalty faithfulness or efficiency if these amounts prescribed in the contracts of employment or the inner workings of the system or facility has been custom or deal by giving workers become so established they consider part of the wage is not a donation.

Employment contract definitely regulates the employment relationship for both the parties.
To the employee it brings a stability to performance improvement(instead of hopping from one job to another) with time understanding the company policy & procedures.
For the employer it gives him time to judge the employee performance well.
However it should be noted that the contract abides by Statutory laws

Employment contract regulates the employment relationship.
It is a formality of an agreement under the law between the employee and the employer signed by both parties.
With the employee contract the parties understand, and that they are obliged to follow the agreement they signed.
Any act of breach is subject for punishment under the law.
In this way, a contract regulates the relationship.

The employment contract is the binding factor between the relationship of the employee and his employer.
Both parties must know their obligations, rights and limitations with the proper guidance of the local laws governing it.
That's why I strongly believe that employers must have a proper and just employment contract for the betterment of both parties.

it puts the dots on the letter as we say in Arabic, business does not work in a vacuums, there where contacts take place, it saves the businesses and work environment from being functioning in a chaos

actually it is all about the labour law of the country you are in, and your employer's implementation of that law in his firm .