It is not an acronym.
There are many different numbering schemes.
On motor contactors, manufacturers in the US usually labeled their incoming connections as L1, L2, L3. And then they will use T1, T2, T3 as the load terminals. In other parts of the world, terminals were labeled R, S, T and U, V, W.
A common current practice is the dual numbering scheme of L1/1, L2/3, L3/5 and T1/2, T2/4, and T3/6
