What is the exact way of using where clause in PostgreSQL 12?

Question

I am trying to query a table using where clause in Postgres 12

case 1

SELECT * 
FROM schema.e_employee_table 
WHERE FK_EMPLOYEE=100;

so when i query the above query i get "FK_EMPLOYEE" column is missing.

case2:

SELECT * 
FROM schema.e_employee_table 
WHERE "FK_EMPLOYEE"=100;

when i query the above one i got the query results

So can someone explain do we need to put our column name inside "",is it the syntax. If so where it is mentioned in the official documentation.

And I can also see the column name we are mentioning is case sensitive.