Restricting a field based on Roles can be easily configured using Perm Level, which is required by most organizations. To define a Perm Level, you can go to the respective form and Customize it.
Let's take a scenario where the organization doesn't want its Employee (Accounts User) to edit the Rate of the item while creating a Sales Invoice. To do that, we can simply make the Item Rate field a read-only.
This is how the Role Permissions Manager will display the newly created Rule with Perm Level as 2:
To know more about Perm Level, click here and for any further assistance, click here.