Appointment Letter For Specific Period

Dear seniors,
I am HR executive of IT company.
We have not issued any appointment letter to senior employees so far as it was verbal agreement between employees and employer.
Now we are planning to issue Appointment letter for them.
As we are IT company, working depends on other companies projects.
Sothere is no Permanent projects in our company.
In this case how we can issue the appointmnet letters to employees.
Can we give appointment letters for specific period?
will it be sort of contact employment and if yes, what will be the conditions should considered while issuing appointment letters?
kindly guide me.

Dear Prajakta,
Appointment letters are issued to an employees on his/her time of joining and which has a probation period and then with certain points which would abide the rules of the organization. You have mentioned that "As we are IT company, working depends on other companies projects.So there is no Permanent projects in our company"
That sentence is little vague as if that is the exact situation, then you shall hire employee on contract rather than on permanent roles.

Dear Prajakta,
There is no concept called as Contract Appointment Letter.It is only a Contract that will be signed as an agreement between the employee and the employer.
Regards,
Munindar