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Actually, -10dBV (the "V" is important, as is the "m" in +4dBm...dB expresses a ratio, and without a reference there is no ratio) is considered line level as well. It can be confusing becasue some instruments do put out a line level signal as well, and the inputs on many devices have enough gain to accept a mic-, intrument-, or line-level signal with no problem. An instrument-level signal is usually considered one that's between a mic- and line-leve signal, such as that put out by an electric bass or guitar, and is usually a higher-impedence output than that of a line-level device as well.
The difference is in the amount of voltage. Instrument level (passive guitar pickups, SM57 microphone, etc.) is decidedly less voltage than is line level (mixer output, line out jack, etc.). Reading the "spec's" of various devices gives one an opportunity to generalize and to form this concept of understanding regarding equipment interconnection.