For safety and security, a uPVC sliding sash window can be fitted with restrictors to prevent the window being fully opened.
With the lower sash closed, locate the metal rectangular fittings positioned on either side of the top sash.
Push the top ribbed section of the button up and the restrictor will pop out. Slide the ribbed section back down to engage it.
To disengage the restrictor, push the top ribbed section of the button up, push the restrictor back into the sash and lower the ribbed button down.