Please look at the following sentence. I wouldn't say this that way and would consider a kind of determiner that should be used before the word "unit".
What about you please?
Sort the telemetry records by unit number.
In this instance, the complement of the preposition by is a category or the name of an attribute and thus no determiner (aka "zero" determiner) is the right choice.
Just as in an apartment complex each apartment has a unit number (they all possess that attribute), the telemetry records in your example have been assigned a unit number.
The students were grouped by age.
She opened the outer doll, and then the doll inside it, and then the doll inside it, until she had all of the dolls before her; she then arranged them by height.