What are you trying to do?
The "Process documents from ..." operators can calculate TF/IDF, Term Frequency, Term Occurences and binary term occurences for you if you select the appropriate option.
The result of "Process documents from ..." is a structure where each token (term) an attribute that has a value for each document. You can use the Loop Attributes operator to calculate something for each attribute.
For example, I used Loop Attributes to sum up the TF/IDF of each attribute (not that it makes sense but it illustrates what you can do).
I added the Loop Attributes after the Process Documents step and set the following options:
Attribute filter type: value_type
Value type: real (as the TF/IDF vectors are real numbers)
Loop Attributes creates a macro that can be used to reference the attribute currently in the loop.
Inside Loop Attributes I added a Generate Aggregations operator with the following options:
Attribute name: sumatt_%{loop_attribute}
Attribute filter type: single
Attribute: %{loop_attribute}
The result is a sum attribute for each attribute (token).