Example implementation algorithm
1. Add a button in the Contacts section
Create a replacing client module of the Contacts section.

In the created client schema, add SelectQueryContactButtonCaption localizable string and set its value to Select contacts.

Add a configuration object with the settings determining the button position to the diff array.
2. Add handler method for the button pressing event
To enable reading the records when the button is clicked, add the following method to the methods section of the replacing client schema:
In the client of the application core, there is not a class like the server core SelectQuery class. To select data from a section, use the Terrasoft.EntitySchemaQuery class. For more information on this class methods and properties are described in the API documentation.