Field level security in CRM 2015

If you have any doubt in the post please post comments. I will try to solve your problem.

The Field Security column helps to quickly identity the behavior of the field towards field-level security by using the statuses below:

• Disabled – Available for field-level security, but is presently not secured by an administrator
• Not Applicable – Not valid for field-level security
• Enabled – Attribute secured by an administrator

Limitation
There is one restriction regarding Boolean attributes to mention here. Since it is necessary to read the result of a Boolean attribute to be either Yes/No or True/False, you will not have the ability to restrict read access. You will only have the ability to restrict create and update access.