Britax Safe-n-sound BS8100A-020133 Series Instructions For Installation & Use page 17

SHOULDERHEIGHTMARKERS  
ANDTHENEXTSEAT
Nexttypeofrestraintthatcanbeusedinthecarforyourchild.
When your child has outgrown the seat, you can either:
Use a booster seat suitable for children up to 8-10 years of age and a
Child Harness suitable for use with the booster seat.
Or, use a booster seat suitable for children up to 8-10 years of age with
the car's seatbelt.
For more information on suitable booster seats visit www.britax.com.au
If a child's legs do not hang over the edge of the vehicle seat then
they need to use a booster seat so that the seatbelt fits correctly and
so the child does not slump. A lap seatbelt on the child's abdomen is
dangerous.
NOTE: This child restraint includes a booster seat configuration for
children 4 years to approximately 6 to 8 years.
17
Instructions for Installation & Use - Series No. BS8100A-020133