The FAA requires (91.159) that a VFR (visual flight rules) flight that is CRUISING higher than 3,000 AGL (above ground level) choose an altitude based on the direction of flight. This helps reduce the risk of mid-air collision.
The rule says: on a magnetic course (desired track) from 000° through 179° magnetic, pick an odd thousand foot altitude and add 500′. If the magnetic course is 180° through 359° magnetic, choose an even thousand and add 500′.
Say my course is 003° magnetic and I’m above 3,000 AGL, I’ll fly an odd altitude plus 500 feet. If I’m over country only 100′ above mean sea level (MSL), I can choose 3,500′ or 5,500′ or 7,500′, etc. If there’s a strong west wind and I’m crabbed 10° to the left to a heading of 353° magnetic, I still should fly odd+500, since the magnetic COURSE is what calls the shots in the US. If the true course is 350°, but the area I’m transiting has a variation of 13° West, my magnetic course is still 003° magnetic, so 5,500′ would be good, but 6,500′ would not.