Need to know the meaning of each element in CAS Policy xml?

Hi,

below is the XML for CAS Policy , can somebody please explain me the meaning of each element in below xml in more simple language.

<CodeAccessSecurity><PolicyItem><PermissionSet
      class="NamedPermissionSet"
      version="1"
      Description="Permission set for custom test WebParts"><IPermission
          class="AspNetHostingPermission"
          version="1"
          Level="Minimal"
        /><IPermission
          class="SecurityPermission"
          version="1"
          Flags="Execution"
        /><IPermission
          class="Microsoft.SharePoint.Security.SharePointPermission,
            Microsoft.SharePoint.Security, version=11.0.0.0,
            Culture=neutral, PublicKeyToken=71e9bce111e9429c"
          version="1"
          ObjectModel="True"
         /><IPermission
          class="System.Net.WebPermission, System,
            version=1.0.5000.0, Culture=neutral,
            PublicKeyToken=b77a5c561934e089" version="1"><ConnectAccess><URI uri="https?://.*" /></ConnectAccess></IPermission><IPermission
          class="System.Security.Permissions.SecurityPermission,
            mscorlib, version=1.0.5000.0, Culture=neutral,
            PublicKeyToken=b77a5c561934e089"
          version="1"
          Flags="ControlThread, UnmanagedCode"
        /><IPermission
          class="System.Security.Permissions.EnvironmentPermission,
            mscorlib, version=1.0.5000.0, Culture=neutral,
            PublicKeyToken=b77a5c561934e089"
          version="1"
          Read="UserName"
        /></PermissionSet><Assemblies><Assembly PublicKeyBlob=PublicKeyBlob /></Assemblies></PolicyItem></CodeAccessSecurity>

thanks

gaurav