The league is a ancient unit of length that doesn't below to a unit system. Its symbol is simply - league.
It was originally an ancient Celtic unit as well as being common in Latin America and meant the distance "a person could walk in an hour". It is no longer a recognised unit in any nation.
Once it started to be quantified, in Argentina (legua) it represented 5.572 km, in England it was quantified as 3 miles and at sea a league is 3 nautical miles (3.452 miles).
The micrometre (American spelling micrometer) is a unit of length and is a multiple of the SI unit metre. It uses the symbol μm.
It represents one millionth of a metre and is also known by its previously depreciated name micron.
In the US, the term 'micron' helped distinguish the unit name from the device 'micrometer' but the International System of Units (SI) discontinued this term in favour of micrometre as the prefix system required the use of the Greek symbol 'μ' which invoked the factor x10−6.