Hi Ishani,
If you are currently using groups in service desk, such that the L1 analysts are on a group and the L2 analysts are in another group, you could attempt to accomplish it this way. First, you would have to create a custom field on the CR object as an SREL to the group object - lets call this field z_group_l2. That new field would need to be set to default to a value of the L2 Analyst Group. Then you can create a data partition constraint such that the assignee would need to be in z_group_l2.group_list.member. This way it would not allow the ticket to be updated if the assignee is not a member of that group. Keep in mind though that this would also prevent that L1 analyst from transferring a ticket to another L1 analyst because the constraint would prevent that as well. Additionally, since this is a customization, you would be responsible to maintain it going forward through patches and upgrades where necessary.
That is the only scenario that we can think of to do what you are looking to do.
Anyone else out there have any ideas?
Thanks
Jon I.