The name suggests a versioned asset pack, likely for software like . In the context of independent animation assets, "solid" usually refers to one of the following:
: In some Japanese animation packs (often found on sites like BowlRoll or Booth), "Solid" can be a metadata tag for a specific pose or a "non-transparent" texture setting.
There is no widely documented or standard technical "solid feature" associated with a file named in major 3D animation or gaming repositories.