Several ways for you to know how to share records in Salesforce

Before you start with developing Sharing rules, be aware of the following forces that will restrict you from sharing records in objects with a users:

1. License Type
2. Profile 
3. Permission Set
4. System Remission

Do you know what is the difference between "Salesforce" and "Salesforce Platform"? When creating or editing a Salesforce User, there is a picklist field that lets you assign a specific User License. 

Salesforce licenses are designed for users who require full access to standard CRM and Force.com AppExchange apps. CRM apps are anything that requires access to Standard Objects like...

• Leads
• Opportunities
• Forecasts
• Cases
• Solutions

Salesforce Platform licenses are designed for users who only need access to custom apps, and NOT the standard CRM functionality. Salesforce Platform users DO have access to the "core" Salesforce Standard Objects and functionality, like...

• Accounts
• Contacts
• Reports
• Dashboards
• Documents
• Custom Tabs

Now the question becomes: How can you ensure data security?