Why does the parser pass 'false' as required to the VerifyNotBefore and VerifyIssuedAt?

[DukeLuke94]

If I configure the parser to validate the IssuedAt claim (with parserOptions: jwt.WithIssuedAt) and I provide a jwt token without an "iat" claim, the token is valid. Why? Because the parser passes false as required, as apposed to the other functions such as verifyAudience and verifyIssuer. What's the logic behind this?

`

if err = v.verifyNotBefore(claims, now, false); err != nil {
	errs = append(errs, err)
}

// Check issued-at if the option is enabled
if v.verifyIat {
	if err = v.verifyIssuedAt(claims, now, false); err != nil {
		errs = append(errs, err)
	}
}

// If we have an expected audience, we also require the audience claim
if v.expectedAud != "" {
	if err = v.verifyAudience(claims, v.expectedAud, true); err != nil {
		errs = append(errs, err)
	}
}

// If we have an expected issuer, we also require the issuer claim
if v.expectedIss != "" {
	if err = v.verifyIssuer(claims, v.expectedIss, true); err != nil {
		errs = append(errs, err)
	}
}

`