Bitcoin layer-2 projects are under scrutiny as experts debate whether they truly deserve the classification. With 73 Bitcoin scaling solutions currently under development, the term "layer 2" is contentious due to the structure of most self-proclaimed Bitcoin layer 2s being more similar to a sidechain. Bai Yu, head of CKB Ecosystem Fund, argues that the term L2 for sidechains is simply a marketing tactic. A true L2 should operate on top of the base layer, inheriting its security, while a sidechain is a separate blockchain with its own security model. This distinction could become crucial if the security of users' funds is threatened by new vulnerabilities of scaling solutions that don't exist on the native Bitcoin chain.