The Conformitè Europëenne (CE) Mark is representative of a set of agreed standards for Safety that are in place across the EU. Manufacturers across the bloc are required to meet these rigorous standards in order for products to be deemed safe for use.
The CE Mark should be visible on the product and the marking needs to be at least 5mm in size so that it can be easily read,
The CE Mark demonstrates that the manufacturer has achieved the necessary standards in their product manufacturing process and that the item - such as your window frame - meets guidelines for safety that have been defined by the EU.
This system will apply until at least 2022, at which point, the UK Government are expected to create an equivalent standard for manufacturing.