看看这种错误消息的一个例子：
combination reference table/field XXX does not exist
引起错误的字段类型为LOSMENGE，字段名为INSLO:
该字段的基本数据类型(Data Type)为 QUAN，长度为13.
根据ABAP帮助文档, QUAN类型需要和某种单位字段结合在一起，用于描述实际业务流程中的数量，比如 一本书，一件啤酒，一捆绳子等等。
按照ABAP帮助文档里的规定，类型为QUAN的字段，必须设置一个类型为 UNIT 的字段作为单位，二者共同描述“数量”这个概念，缺少单位，数量单位的语义就不完整。
For every structure component of data type QUAN, a component of the same structure or of a different structure, database table, or view must be specified in the individual editing of the component as a reference field, which has the data type UNIT. This field is the unit key which determines the unit of the quantity. From a technical perspective, the data type UNIT is handled like a type CHAR with length 2.
