The Issue Picker custom field allows you to select (automatically or manually) the issues similar to the current one as having the same value in a custom field. There is also an ability to create a JQL-filter to limit the list of the issues available for selection.
After creating new custom field you can configure it by clicking Configure → Edit Set up CRM issues picker field:
The issues with the same value in this custom field will be available in the Issues Picker field.
JQL-filter (if specified) will limit the list of issues available in the Issues Picker field.
Filled with options by default
if checked, all issues that meet the conditions will be auto-selected in the Issues Picker field. Otherwise, the issues will be available for manual selection.
Related custom field
An extra parameter to be displayed for the selected issue beside issue key and summary.
Then you can use a new JQL-operator for search issues that are selected in this Issue Picker field of other issues, like:
id in MentionedIn("Deals", "project = CRM")
"Deals" - is a name of IssuePicker field
"project = CRM" - additional JQL filter to the parent issue
So it means - find the issues, that are selected in a field "Deals" of issues of project "CRM".
How it looks in Issue screen: