$$ q\left(\mathbf{x}_{1: T} \mid \mathbf{x}_{0}\right)=\prod_{t=1}^{T} q\left(\mathbf{x}_{t} \mid \mathbf{x}_{t-1}\right) $$
What dose the $\mathbf{x}_{1: T}$ mean? This is from https://drive.google.com/file/d/1noQ2d4-nzSh3Yp3-mOOYN8YGJoSa4pmz/view page 17.