I know this is a very old question but I was looking for it and couldn't find anything.
So if someone comes here like me here is the answer.
The values are defined in lib\grade\constants.php and are:
// Category aggregation types
/**
* GRADE_AGGREGATE_MEAN - Use the category mean for grade aggregation.
*/
define('GRADE_AGGREGATE_MEAN', 0);
/**
* GRADE_AGGREGATE_MEDIAN - Use the category median for grade aggregation.
*/
define('GRADE_AGGREGATE_MEDIAN', 2);
/**
* GRADE_AGGREGATE_MIN - Use the category minimum grade for grade aggregation.
*/
define('GRADE_AGGREGATE_MIN', 4);
/**
* GRADE_AGGREGATE_MAX - Use the category maximum grade for grade aggregation.
*/
define('GRADE_AGGREGATE_MAX', 6);
/**
* GRADE_AGGREGATE_MEDIAN - Use the category mode for grade aggregation.
*/
define('GRADE_AGGREGATE_MODE', 8);
/**
* GRADE_AGGREGATE_WEIGHTED_MEAN - Use a weighted mean of grades in the category for grade aggregation. Weights can be manually set.
*/
define('GRADE_AGGREGATE_WEIGHTED_MEAN', 10);
/**
* GRADE_AGGREGATE_WEIGHTED_MEAN2 - Use a simple weighted mean of grades in the category for grade aggregation.
*/
define('GRADE_AGGREGATE_WEIGHTED_MEAN2', 11);
/**
* GRADE_AGGREGATE_EXTRACREDIT_MEAN - Use the category mean for grade aggregation and include support for extra credit.
*/
define('GRADE_AGGREGATE_EXTRACREDIT_MEAN', 12);
/**
* GRADE_AGGREGATE_WEIGHTED_MEAN2 - Use Natural in the category for grade aggregation.
*/
define('GRADE_AGGREGATE_SUM', 13);