In the context of a three-phase transformer, 1U, 1V, and 1W typically refer to the terminals of the high-voltage (primary) winding,

 In the context of a three-phase transformer, 1U, 1V, and 1W typically refer to the terminals of the high-voltage (primary) winding, while 2u, 2v, and 2w refer to the low-voltage (secondary) winding. The lowercase letters (u, v, w) usually indicate the lower voltage side, and the corresponding uppercase letters (U, V, W) indicate the higher voltage side. They represent the phase terminals (R, Y, B) of the windings. 

Here's a more detailed breakdown:

1U, 1V, 1W: These denote the three terminals of the primary (high voltage) winding. 

2u, 2v, 2w: These denote the three terminals of the secondary (low voltage) winding. 

1N, 2N: If present, these are the neutral points of the primary and secondary windings, respectively. 

The letters (u, v, w) and (U, V, W): These are used to identify the phase sequence (e.g., R, Y, B) of the windings. The lowercase letters typically represent the low voltage side and the uppercase letters the high voltage side. 

Vector Group: These markings (e.g., Dyn11) are part of the transformer's vector group designation, which describes the phase relationship between the primary and secondary windings. 

For example, a Dyn11 transformer means the secondary (wye side) lags the primary (delta side) by 30 degrees.