Custom fields as the name suggests are the fields that make it possible to define additional properties for an entity.
In Test Collab the custom fields can be defined for:
Test Cases
Test Executions
Custom fields can also be seen as additional parameters that refine the arguments between the person defining the Test Execution and the person who actually executes it. They may be set to control the execution of a test.
If custom fields are present under the project, then they can be used in following ways:
To define additional properties to a test case and test execution under that project
To map their values with the values of issue manager fields that are selected while configuring issue manager for project so that the reported issues hold the mapped custom field's values
To define the skip conditions for an individual test case or a suite altogether. While defining test suites and test cases, a user can select a custom field of type "Option box" and "Multiple Choice Option box" to be used for a condition and select the value for it to drive that condition.
Control the execution, as they may be the part of skip conditions
As source of communicating a fixed set of information between a tester and others
As part of the analytical data
Any number of custom fields can exist under a project.
Following are the Properties of a custom field:
Custom field defined for
Name
Label (shown to users using it)
Type (to govern the user interface)
Required (whether it will be mandatory or not)
Default value
Adding, Editing, Deleting of custom fields can only be done by an administrator.
Add a Custom Field
Following is the form used to enter a new field's details.

The user will be required to enter/select the following:
The entity for which custom field is being defined
The name for the new custom field
Its label
Its type, where type can be
Text
Multiline Text
Option Box
Multiple Choice Option Box
Date
Whether it will be required to enter data for the field, or not
Its
Default value (for Text), or
List of options (for Option Box/Multiple Choice Option Box) available
Show on search option can be selected, if this field will be required to be used as a filter for search criteria.
Custom field for, name, label, and type are mandatory.