Question about "Main" datasource for a form

Hi everyone, 

As the title shows, I have the following 2 cases:

-1- Form [CustParameters], go to [Designs] > [Design], look at Properties "DataSource", it's empty.

-2- Form [BankTrans], go to [Designs] > [Design], look at Properties "DataSource", it's set to be "BankAccountTrans"

Someone is saying that if this "DataSource" property has a value, the corresponding data source is the main data source for the form. Is this true? What does this "main" data source mean? What difference it makes when retrieving the data if setting or not setting this property?  When should I set this property, as some forms have this property set, while other forms leave it as empty?

Thank you,

Kwen