Length Context allows you to read the length of a path or paths and pass values through to affect any other attribute.
Remapping - remap the incoming values.
None - do not remap values.
Number Range - see Number Range.
Number Range to Color - see Number Range to Color.
Create some Trails as per the example usage on the Trails page.
Add a Random Behaviour to trails.length
Set the Random Behaviour's Minimum to 10 and the Maximum to 200.
Create a Length Context.
Source Minimum to 0
Source Maximum to 200
Minimum to 1
Maximum to 40
Connect lengthContext.id > trails.strokeMaterial.width
As the Trails grow in length their Stroke Width will increase (1px when they have a length of 0, 40 at 200).
Try a similar thing with Color (adjust the remap values accordingly) or Connect Shape.