There is no required documents for issuing CoE. Companies usually use the CoE as a bargaining chip for employees to process their clearance since once an employee does have their CoE, they don't care or don't have motivation to complete their clearance form.
However, there is no rule when a company can issue the said CoE, and like processing final pay, it can be released in a reasonable time.
For CoE, the content is usually what is stated on record. ideally, the employee will try to keep a good work record so that there would be no bad record to reflect during their background investigation or on CoE.
What some companies do is put in their CoE: "As per Company Records," and "until her date of AWOL on xx/xx/xxxx and have not been cleared from her liabilities" so that future companies will know her reason for separation and to force separated employee to process their clearance first before getting a favorable CoE.
hope this helps