Echo SRM-2100SB, SRM-2400SB Operator's Manual page 16

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8 Tooth Weed/Grass Blade P/N 69600120331 is designed for grass,
garden debris and thick weeds. DO NOT use this blade for brush or
heavy woody growth, 3/4" in. (19 mm) diameter or larger.
Brush/Clearing Blade P/N 69500120331 is designed for cutting brush
and woody growth up to 3" in. (76 mm) diameter.
Use Shoulder/Waist Harness P/N 99944200200 The Shoulder/Waist
Harness when used in a trimming operation with nylon line head
suspends the trimmer from the operator's shoulder and reduces operator
fatigue.
During blade operation, the same fatigue reduction is achieved. Safety
to the operator is also enhanced by preventing blade contact with the
operator's hands and feet by restricting trimmer movement.
NOTE
In case of Emergency, pull up on the Quick Release Collar to
disconnect the trimmer from the harness.