One of the issues I had noticed earlier with Business process area in Dynamics CRM 2013 during CRM 2013 Rollup 1 was that business process area didn’t take the fields in CRM 2013 Form (specially on load) into consideration while showing field as enabled/disabled, visible/not visible etc. I find the same issue still existent with CRM 2013 SP1 Rollup 1. What I am trying to say is say we need some fields as disabled on the form, CRM 2013 Business process area does not take it into consideration and Business process area is still editable. Look at the scenario in below screen. Here we have the same field in two places, one in the business process area and one on the form, highlighted in red rectangle below. You can notice that the field is disabled on form, but still enabled in Business Process area which just breaks the disabled field in the form:

For Scenarios like this, following a good workaround:

We need to create a business rule without any condition, Refer screen below:

Once this is applied, the field will be locked in both places (Marked in green rectangles):

This is extensible in sense that as we need more disabled fields, we just need to keep it adding to the business rule.

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Hope it helps!