Newbe with old Questions

[GovB]

New to "O" scale and would like to know what the difference is between items described as "Scale", "Semi Scale" and others with no scale reference and why?

Thanks, GovB

[phillyreading]

Some companies make stuff to a scale of 1:48, which is closer to true scale for O gauge, other companies make stuff between 1:43 and 1:54 scale or off scale. 
Semi scale(usually means below scale) and can have different meanings with different companies.
Some of the larger offerings like the UP big boy(6-8-8-6) and the articulated steam or tri-plex are closer to 1:52 scale to make the engine run on O gauge track. If you had the UP big boy at scale it would need about 096 curves to operate.

I hope this helps.

Lee F.