Header：上图蓝色和浅绿色文字。
A header is a single line defining a mapping of the following value lines to the type system. A header applies to all processed value lines until the next header or until the end of file, whichever comes first. You can put any number of headers into a CSV file.
定义了接下来value line和type系统的映射关系。
格式：
mode type[modifier=value];attribute[modifier=value];attribute[modifier=value];attribute[modifier=value][;...];attribute[modifier=value]
方框里的modifier：Gives additional information for translating a value record to the mapped type attribute
例子：INSERT_UPDATE
category;code[unique=true];name[lang=de];name[lang=en];$supercategories;$thumbnail;description[lang=de];order
This header states that each following value line creates or updates a category instance, until another header occurs. 注意，大小写无关。
INSERT Address;firstname;owner(Principal.uid|AbstractOrder.code)
;Hans;admin
;Klaus;O-K2006-C0000-001
由|分隔开的语法叫alternative pattern.
When importing the first value line ;Hans;admin, the ImpEx extension searches all instances of the Principal type to verify if any instance exists where the uid attribute is set to admin.
Impex extension首先查找所有principal实例，看是否存在其uid属性值为admin的实例。
注意我们现在是试图插入Address数据，如果上述的搜索命中，则将Impex语句里的address值维护到搜索命中的admin Principal实例去。
If an instance exists (and there is, by platform default), the new instance of the Address type has its owner attribute set to a reference to that Principal type instance.
如果并没有Pricipal实例，其uid属性等于admin，那么进行第二轮搜索，根据AbstractOrder的code字段进行搜索。
Attribute modifier：
...;attribute[modifier=value,modifier=value,modifier=value];...
或者
...;attribute[modifier=value][modifier=value][modifier=value];...
The value of the $catalog macro sets the catalog id. The value of the $version macro sets the version string of the catalog version to export.
$catalog=clothescatalog
$version=Staged
宏的实现：
$catalog=catalog(id)
$catalogVersion=catalogVersion($catalog,version)