I'm going to use couple examples to ask my question:
Looking at KBVY chart supplement, it says:
RADIO AIDS TO NAVIGATION: NOTAM FILE LWM. LAWRENCE (L) DME 112.5 LWM Chan 72 N42°44.43 ́ W71°05.69 ́ 140° 12.3 NM to fld. 306/0W. DME unusable: 160°–195° byd 23 NM blo 2,500 ́
Looking at KMLT chart supplement, it says:
RADIO AIDS TO NAVIGATION: NOTAM FILE MLT. (H) (H) VOR/DME 117.9 MLT Chan 126 N45°35.20 ́ W68°30.93 ́ 317° 8.0 NM to fld. 553/20W. DME unusable: 330°–350° byd 35 NM blo 6,000 ́ VOR portion unusable: 330°–340° byd 35 NM blo 6,000 ́
Referring to FAA Chart Supplement guide:
KBVY is 12.3 NM from KWM-DME on the bearing of 140°. What does
0w variation mean?
KMLT is 8.0 NM from MLT VOR on the bearing of 317°. What does
20W variation mean?
KBVY is close to the middle of 14°W and 15°W isogonic lines, and KMLT is somewhere between 16°W and 16°30'W isogonic lines, closer to 16°.
I know what magnetic variation is, and how to use it to convert true course to magnetic course. What I don't understand is what is it implying here, in the context of chart supplement, and how one would use it, for what purpose?
It clearly doesn't correlate with the isogonic lines in that area, and the bearings given are magnetic so they shouldn't need conversion (see here: What are the units for the bearing to center of an airport from radio aids to navigation in chart supplement?).