Further docs on levels

docs/reference/level_index_id_number.rst

@@ -6,19 +6,25 @@ 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. 
+DataFrame index for each electronic energy level. Different for each ion. 
+Third index for energy levels after atomic number and ion charge. Used in GFALL,
+Chianti and CMFGEN levels DataFrames.
+
+For photoionization cross-sections, this is used to determine the lower and upper
+levels of cross-sections.
 
 =============
 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.
+ionization cross sections, the macro atom and electronic energy levels. 
+Computed from level index. Typically has an upper and lower form to connect levels
+together.
 
 =============
 level_number
 =============
 
-DataFrame index for each level suitable for TARDIS use. Connects upper and lower
-energy levels.
+DataFrame index for each electronic energy level suitable for TARDIS use. 
+Connects upper and lower energy levels.