...
copy values from repeating source issue, e.g $sourceIssue.summary
calculate dates, e.g. $repeating.date.addDays(2).withHours(10).withMinutes(0)
calculate any values using conditions, e.g. #if( ${repeating.date.month} == 10)it's true!#{else}it's not!#end
copy current custom field value (for custom field inputs only): $!sourceIssue.getCustomFieldValue($this)
Please read Velocity references in field values documentation to learn the velocity context and available references in Repeating Issues app templates.