Clarification of Notice Period - LOI is being issued before DOJ without mentioning any terms of employment

1. Conditions : "Notice Period of 3 months is required in case of resignation submitted by employee & company is liable to terminate the service of employee by only one month notice."
Query: Is this is ok and acceptable.
2. LOI is being issued before DOJ without mentioning any terms of employment.

Whether termination or resignation, notice period should be same duration.
LOI (Letter of Indent) or provisional Offer letter could be issued to a person
without mentioning any terms of employment. The purpose of issuing such
letters is to give the candidate about selection confirmation and to enable him/her
to submit his/her resignation to his/her current company. Detailed Offer-cum-
Appointment letter could be issued to him/her after his/her joining the company.
My company follow this practice.