[1] A non-combining form is U+02CD ˍ MODIFIER LETTER LOW MACRON.
The difference between "macron below" and "low line" is that the latter results in an unbroken underline when it is run together: compare a̱ḇc̱ and a̲b̲c̲ (only the latter should look like abc).
Used in Tlingit and Haida (among other Pacific Northwest languages) for the voiceless uvular stop [q].
Close to Korean ㄲ kk; closest English "shocking" Used optionally in the K-dialect of Māori in the South Island of New Zealand, where an original ng has merged with k. The ḵ indicates that it corresponds to ng in other dialects.
There is no difference in pronunciation between ḵ and k. Note that the Unicode character names of precomposed characters whose decompositions contain U+0331 ◌̱ COMBINING MACRON BELOW use "WITH LINE BELOW" rather than "WITH MACRON BELOW".