Column settings allow for options to be set for individual columns of data from the database.
...
Type - the data type, such as number, string, date, etc. - what is this used for? - data validations on INSERT and (more importantly) - indexing, so you can index a text column as string, and a number column as number, so you can use the correct operators when searching with JQL.
Default Value
PK - the column is the primary key in the connected database.
...