Adds docs for various similar level columns

Also moved notation to a more accessible spot.

docs/index.rst

@@ -11,6 +11,7 @@ Carsus is a package to manage atomic datasets. It can read data from a variety o
     installation.rst
     quickstart
     development/compare_atomic_files
+    reference/notation.rst
 
 .. toctree::
     :maxdepth: 2
@@ -26,6 +27,13 @@ Carsus is a package to manage atomic datasets. It can read data from a variety o
     io/vald
     io/barklem2016
 
+.. toctree::
+    :maxdepth: 2
+    :caption: Reference
+    :hidden:
+    reference/notation.rst
+    reference/level_index_id_number.rst
+
 .. toctree::
     :maxdepth: 2
     :caption: Research with Carsus
@@ -50,13 +58,6 @@ Carsus is a package to manage atomic datasets. It can read data from a variety o
 
     api/modules
 
-.. toctree::
-    :maxdepth: 2
-    :caption: Legacy Code
-    :hidden:
-
-    legacy/index.rst
-
 
 .. seealso::
     Need help? Contact us `on Gitter <https://gitter.im/tardis-sn/carsus/>`_.

docs/reference/level_index_id_number.rst

@@ -0,0 +1,24 @@
+***************
+Differences between level_index, level_id and level_number
+***************
+
+=============
+level_index
+=============
+
+DataFrame index for each electronic energy level. Different for every level. 
+Third index for energy levels after atomic number and ion charge. 
+
+=============
+level_id
+=============
+
+Global cross-matching ID for levels across multiple DataFrames i.e. between photo
+ionization cross sections, the macro atom and electronic energy levels.
+
+=============
+level_number
+=============
+
+DataFrame index for each level suitable for TARDIS use. Connects upper and lower
+energy levels.