The share of samples (0 to 1) that carry the value it rests on; null where samples don't come into it.
The edits the activity went through (empty when unedited).
How it is worked out, in a sentence.
A short name for the source: 'gps', 'device', 'pool-lengths',
'samples', 'file', 'recorded', 'derived', … or 'none' when the
activity doesn't have it.
Where one number of an activity comes from.