1 - says whether or not you will allow a user to go in and edit (add, delete, modify) any records in any EvoERP database.  It is a *very powerful* utility and access should not be granted lightly, as you can end up trashing your data if you use it incorrectly.
 
2- From the EVoERP help file:
"The Security Level [1-999] is used in the future Evo-ERP lookup drill-down grids to control which user can see and/or change which data.  Level 1 is the administrator level that can see and edit all data and level 999 is the least privileged user.  It is not yet used."
 
3 - From the EvoERP help file:
"The Security Code controls what menu access in Evo-ERP a user will have if no user-defined Menu exists for the user name.  A=Admin; P=Power User; 1=Sales Rep 1; 2=Sales Rep 2; C=Customer; V=Vendor; U=User.  If nothing is defined then User will be used...."
 
An "A" user will have access to every single menu item there is.  P a little less, etc.  I don't know what exactly each has access to after A.  Maybe Lynn or Allen can help there.  
 
Val