You can now map custom activity fields to any message and field, not just session fields. The default behaviour is to look in the session record for a matching field name or number. However you can prefix the field name/number with the name/number of the record to search in other records.
Examples:
activity.total_timer_time-
140.4(field number 4 of message number 140, Garmin Venu watch training effect times 10)
The script of the custom field can be used to transform the value once it has been read:
activity.isNew ? activity.TrainingEffect / 10 : activity.TrainingEffect
This expression only does this when the activity is new or its file is reprocessed otherwise it returns it as is.