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:
|Custom 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: