Lots of email messages about program memberships describe the points earned. I propose adding a membershipPointsEarned property to ProgramMembership.
The markup might looking something like:
{
"@context": "http://schema.org/",
"@type": "ProgramMembership",
"name": "ACME customer rewards",
"membershipPointsEarned": {
"@type": "QuantitativeValue",
"name": "Starpoints",
"value": 500
}
}
The expected type of QuantitativeValue is to allow authors to use their custom name for the point system.