I'm trying to help my student to translate one of the manuals which she needs for work. There is a sentence that I don't understand completely:"For example, you might want to track the license number for automobiles, but the square footage for buildings". To be honest, I don't see a point of the "but" at all. It describes one of the functions of the program, so it's the main context here. Please, can anyone explain what is this "but" for and how it changes the meaning of this sentence? Thanks in advance.
As Mick notes in his astute comment, the use of the conjunction points up a contrast in the clauses.
To simplify the construction, let's reduce it to more common terms.
I drink coffee in the morning, but I drink wine at night.
I drink coffee in the morning but wine at night.
Here the speaker contrasts a morning beverage with an evening one. This emphasizes the uses for each: one for waking up, the other for winding down.
Note that a different conjunction can be used as well:
I drink coffee in the morning, and I drink wine at night.
I drink coffee in the morning and wine at night.
Here the contrast is allowed to emerge, if it can, from the organic elements of the sentence. If the terms have any natural contrast, as coffee and wine appear to, then a contrast is expressed whether or not it is emphasized by means of the conjunction.
Suppose there is no innate contrast in the terms.
I drink milk in the morning and fruit juice at night.
The statement becomes neutral, a distinction that relies merely on time expressions to set the terms apart. The beverages could be reversed without suggesting any intrinsic contrast between them, and the reader or listener could suppose that the choice is simply your custom. If the speaker uses but to coordinate the clauses, however, we are being asked to entertain the possibility that there is a reason for drawing the distinction:
I drink milk in the morning but fruit juice at night.
The distinction still may not be obvious (as it would be in the case of coffee and wine), but this sets up the contrast for further discussion:
I drink milk in the morning but fruit juice at night. Milk calms my stomach in the morning, while the sugars in the juice keep me from overeating at supper.
The reasons are unimportant, and and could well be used to coordinate the clauses here as well, but but takes us more firmly by the hand and leads us in that direction.