On the majority of Focusrite interfaces, combo XLR/jack inputs accept both microphone and line-level signals. In either case, the signal passes through the variable gain preamp circuit, meaning the gain knob will affect your input level regardless of what you have connected.

This does not mean the signal will be coloured or degraded. Focusrite preamps are designed to be highly transparent and do not colour the signal passing through them (unless exceeding the maximum input).

To work out where you need to set the gain level, please see the following article, which applies to both line and mic level inputs: How high should I set the gain control on my interface?

ISA C8X behaves differently from other Focusrite interfaces in an important way. Its dedicated TRS line inputs completely bypass the preamp and variable gain stage, routing the signal directly to the converters. This means the gain control has no effect when using these inputs. 

Some devices feature fixed gain line inputs (such as 4i4 and Clarett 2Pre). While these don't pass through a variable gain stage, the signal path does include fixed gain amplification.

The following table shows how line inputs are handled across the Focusrite range, from Scarlett 1st Gen onwards;