Q: What does a status change from "With Editor" to "Decision in process" mean in case of my letter to the editor?

Letters to the editor are generally not sent for external peer review. The editor usually reviews these himself/herself and arrives at a decision. In your case, the status changing from "With Editor" to "Decision in process" implies that the editorial review is probably done and the editor is now making a decision on whether to accept or reject it.

The status date changes whenever someone accesses your database from the backend. The status date change implies that the editor checked your manuscript once again. This could be because he/she wanted to take another look at it while making the decision. Don't worry, it will not affect the decision in any way.

Related reading:

• Why does the status date change while the status remains 'With Editor'?
• Why does the status date of a submission change periodically though the status remains unchanged?