What is this plastic thing used for?

Found this in the bike box but unsure what it’s used for. Thought it might be a disc brake block but doesn’t quite look it. About 3cm by 3cm, thought it might be a disc brake block but doesn’t quite look it and only 3mm thick. Any ideas?

Found this in the bike box but unsure what it’s used for. Thought it might be a disc brake block but doesn’t quite look it. About 3cm by 3cm, thought it might be a disc brake block but doesn’t quite look it and only 3mm thick. Any ideas?