Hi:
_N_ and _ERROR_ are automatic variables created for every DATA step program. _INFILE_ is created as an automatic variable that allows you to access an INPUT buffer.
Here's the doc:
http://support.sas.com/documentation/cdl/en/lrcon/62955/HTML/default/viewer.htm#a000695104.htm
http://support.sas.com/documentation/cdl/en/lrdict/64316/HTML/default/viewer.htm#a000146932.htm
_TYPE_ could be created by a number of different procedures. For example, PROC MEANS creates an output dataset with _TYPE_ to indicate the combination of CLASS variables used to create a particular observation. Other procedures use _TYPE_. You will have to look in the doc for the procedure that created _TYPE_ to find out about that.
BY group processing creates a different type of automatic variable, for example. If you have this statement in a program:
[pre]
data myclass;
set sashelp.class; by age;
...more code...
run;
[/pre]
Then you can use the automatic variables FIRST.AGE and LAST.AGE to detect the beginning and ending observations in a group.
Usually, if you search on the support.sas.com site, you can limit your search to the SAS documentation and this will help you zero in on the doc for the feature, option, etc that you are looking for -- it is more concentrated than a Google search -- and for documentation, generally more effective.
cynthia