‘Alpha’ is generic name still used by Sony for all its interchangeable lenses, but it also refers to the older Alpha range of SLT cameras.
This can be confusing. Sony has made cameras in two types – SLT (single lens translucent) and mirrorless models. Both are Alphas, but the Alpha A9 (mirrorless) and Alpha A99 II (SLT) are entirely different cameras with different lens mounts and lens ranges. Sony’s SLTs use Alpha A-mount lenses, while the mirrorless models use E-mount lenses.