The length of centromeres on both sides of chromosome is different, which is divided into long arm (denoted by Q) and short arm (denoted by P).
The number before the letter indicates the chromosome number, (people have 23 pairs of chromosomes, numbered 1-22, and two sex chromosomes, X Y).
Chromosomes are divided into several "zones", and each zone is divided into several "bands".
The number after the letter indicates the area code. If there is a decimal point after the area code, the number after the decimal point represents a symbol.
The example you gave has no number.
"18q2 1" means "long arm 2 1 region of chromosome 18".
"18p2 1.4" means "chromosome 18 short arm 2 1 region 4".
In some biological papers, it may be described as follows: "An oncogene is located in" 18p2 1.4 ",that is to say, the oncogene is located in the 4 region of chromosome 18 short arm 2 1".