Field level security is a method that allows you to limit access to certain fields by either Team association or by individual users. This will allow you to hide or restrict access to data in sensitive fields such as credit card or social security numbers.

ACT! field level security has three permission levels:

• Full Access – This is the default option for all fields when created. It allows users to have full control of these fields limited only by their assigned Security Roles in ACT!.*
• Read Only -Allows for these users to view this data but can not change it. The field background is gray.
• No Access(available in Premium versions only) – These fields will appear as empty with a gray background.

Full Knowledge Base Article (Answer ID 19185): http://kb.act.com/app/answers/detail/a_id/19185/kw/Field%20security