Answer
List Attributes are FME's way of allowing multiple values per attribute.
Transformers often create a list automatically as part of the output from their particular process, usually when attributes from various different features are grouped into a single feature.
FME lists can be displayed in Inspector or using the Logger transformer. Typical lists look like:
mylist{0}
mylist{1}
There are several transformers that can be used to manipulate lists - see the Lists category in FME Workbench transformer gallery.
Few, if any at all, output formats offer support for lists, so if you want to output the data to such a format you'll need to deal with the list first.
It's recommended to remove lists, especially ones with many items, as soon as you are done with them. This will save memory and processing time and will reduce the clutter of your workbench.
Lists are indicated in workbench by their name, followed by a pair of curly brackets, e.g. mylist{}. A specific list element has its number between the brackets, e.g. mylist{0}.Complex lists can have mulitple elements: mylist{0}.myattribute
Not all list elements have attributes, sometimes a list element simply has a value, for example a list generated by the
AttributeSplitter
In order to access specific list elements in the Workbench GUI, right click on a list (e.g. mylist{}) in the attribute list and choose "Expose elements". Then type in the number of elements you want to expose. Keep in mind that list elements start counting at 0, so exposing 1 element of mylist{} will result in the extra attribute mylist{0} being available.
List elements can have other lists as part of their attributes. mylist{}.secondlist{}
ListConcatenator can be used to convert an FME list to a comma separated attribute value, i.e. :
mylist{0} A
mylist{1} C
to
attribute_list A,C