请问下ContractData类型 字段的意思 例如 size 是指的什么？ _option_strike_ 是指的什么呢？ option_strike，option_underlying，等等都代表什么意思呢？
class ContractData(BaseData):
"""
Contract data contains basic information about each contract traded.
"""
symbol: str
exchange: Exchange
name: str
product: Product
size: float
pricetick: float
min_volume: float = 1 # minimum trading volume of the contract
stop_supported: bool = False # whether server supports stop order
net_position: bool = False # whether gateway uses net position volume
history_data: bool = False # whether gateway provides bar history data
option_strike: float = 0
option_underlying: str = "" # vt_symbol of underlying contract
option_type: OptionType = None
option_listed: datetime = None
option_expiry: datetime = None
option_portfolio: str = ""
option_index: str = "" # for identifying options with same strike price
