Where to find the Market Data fields?

[Tsopic]

subscription_data = {
    'conid': gc_futures[0]['conid'],
    'fields': [
        FIELD_BID_PRICE,
        FIELD_ASK_PRICE,
        FIELD_BID_SIZE,
        FIELD_ASK_SIZE,
        FIELD_LAST_PRICE,
        FIELD_VOLUME,
    ]
}

Saw this example in one of the issues, I've been digging in docs and unable to understand how to bind correct market data fields for the subscription request

Where do I find these values for fields?