Cal. Labor Code 2922 provides that an employer or employee can terminate an employment relationship at any time, for any reason, or for no reason at all. This is known as the "at will" employment doctrine. There are exceptions to this doctrine, the principal ones being where a written employment contract exists with a specified termination date (rare, except for highly-compensated executives), violations of public policy (jury duty, witness subpoena, report of employer's criminal activities, etc.), or unlawful discrimination based upon race, color, nationality, religion, sex, age or disability.
Other than those exceptions, the employer can terminate an employee, and there is no requirement to pay any severance, except in the case of an employee terminated as part of a mass layoff of over 50 persons in a 30-day period, in which case, at least 60 day notice is required, or alternatively, 60 days wages.
Concerning the employer's reasons for termination, and employee is legally entitled to review his or her personnel file -- however, most large employers do not state the reason for a termination in the personnel file -- and fortunately, most employers do not share the reason for a termination with future prospective employers, because of the risk of legal action by the employee for "blacklisting" (wrongful interference with prospective economic relations).
If the employer has offered you a severance package, it usually is in exchange for your agreeing to not sue the employer for unlawful discrimination, especially for age discrimination. If you are 40 plus years of age, then the employer must provide you with specific time period to review the document before signing and a further time period to cancel your agreement after you sign. Every employer is aware of these requirements, so I would be astonished if your severance package does not contain them.
Not sure what else you would like to know, but I think that pretty much covers your original question.
Hope this helps.
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