The 90 days notice period in IBM is mandatory. It is at your manager’s discretion to allow you a 30 days notice period, depending on the situation or a genuine requirement. However, if a person is involved in an active project, they may not accept a shorter notice period. The person would be required till the completion of the project or, until certain milestones are met. The decision about the notice period would be indicated in the "Notice Period Action" field in the mail received by you from the HR at IBM. Since it mentions 90 days being approved, you would have to maintain the required notice period.