Union tool properties. Assignment a value in the field

[ROMANFILOZOF]

Hi,
Please explain me somebody what are the principles of assignment a value to the features that are created after Union tool execution.
The situation is next:
I have two layers with field "weight", the values are different but not "0". When I make Union with one of them and the third layer new feature is created. That feature has a value "0" in the field "weight". When I make Union with the other of them and the third layer new feature is created. That feature has a value "1,4415" (actually doesn't matter, but not "0"). It's a really problem for me to understand that 🙂

[NobbirAhmed]

Here is an example where I have two feature classes input_1 and input_2.

Input_1 has 6 polygons - labelled with object IDs 1, 2, 3, 4, 5, 6 and colored yellow.
Input_2 also has 6 polygons - labelled with object IDs 23, 24, 25, 26, 27, 28 and colored  green.

[ATTACH=CONFIG]15089[/ATTACH]

Between them, 3 polygons from each of the inputs overlap (leaf green - 1, 3, 4 from Input_1 and 24, 27, 28 from Input_2). Thus the output will have 9 features as Input_1 OIDs 1, 3, 4 and Input_2 OIDs 24, 27, 28 are unioned into 3 features.

Each input has a field called Weight - these fields will be transferred to the output. In the output table, Input_1 (yellow) has no overlapping features (with OIDs 23, 25, 26) from Input_2 and thus has no corresponding attribute value for Weight field. As the Weight values are coming from Input_1, of the 9 output rows, these 3 rows will have 0 value (which means no value in this case).

The Weight field carried over from Input_2 to the output will be named Weight_1 (although field label will be Weight) as there cannot be two fields with same name. Input_2 has no overlap with 2, 5, 6 of Input_1 and thus no corresponding feature in the output. Weight_1 values for these 3 rows will have a value of 0 as well.

[ROMANFILOZOF]

Thanx, I see the rule here and the logic is understandable.
But my situation is different.
1. My Input_2 HAS NO field "Weight".
2. So in output only one "Weight" field is created.
3. The places where the overlapping is keep the values of Input_1 (where is "Weight" field with values).
4. The part of Input_2 that is not overlapped is the same layer but another feature and the value for this feature in "Weight" field is "0" with one Input_1 and not "0" with another Input_1.

And that's a problem 🙂