Please read the following table in HTML format from a webpage and then answer the question in detail according to the table. Each table cell is defined by a <td> and a </td> tag. Each table row starts with a <tr> and ends with a </tr> tag. 'rowspan=m' and 'colspan=n' represents a cell spaning over 'm' rows and 'n' columns.

Webpage title:
X.500
Table title:
List of X.500 series standards
Table:
<table id='hittask' contenteditable='False'>
<tr> <th colspan=1 rowspan=1 > ITU-T number </th><th colspan=1 rowspan=1 > ISO/IEC number </th><th colspan=1 rowspan=1 > Title of Standard </th></tr>
<tr> <td colspan=1 rowspan=1 > X.500 </td><td colspan=1 rowspan=1 > ISO/IEC 9594-1 </td><td colspan=1 rowspan=1 > The Directory: Overview of concepts, models and services </td></tr>
<tr> <td colspan=1 rowspan=1 > X.501 </td><td colspan=1 rowspan=1 > ISO/IEC 9594-2 </td><td colspan=1 rowspan=1 > The Directory: Models </td></tr>
<tr> <td colspan=1 rowspan=1 > X.509 </td><td colspan=1 rowspan=1 > ISO/IEC 9594-8 </td><td colspan=1 rowspan=1 > The Directory: Public-key and attribute certificate frameworks </td></tr>
<tr> <td colspan=1 rowspan=1 > X.511 </td><td colspan=1 rowspan=1 > ISO/IEC 9594-3 </td><td colspan=1 rowspan=1 > The Directory: Abstract service definition </td></tr>
<tr> <td colspan=1 rowspan=1 > X.518 </td><td colspan=1 rowspan=1 > ISO/IEC 9594-4 </td><td colspan=1 rowspan=1 > The Directory: Procedures for distributed operation </td></tr>
<tr> <td colspan=1 rowspan=1 > X.519 </td><td colspan=1 rowspan=1 > ISO/IEC 9594-5 </td><td colspan=1 rowspan=1 > The Directory: Protocol specifications </td></tr>
<tr> <td class="highlighted"  colspan=1 rowspan=1 > X.520 </td><td colspan=1 rowspan=1 > ISO/IEC 9594-6 </td><td class="highlighted"  colspan=1 rowspan=1 > The Directory: Selected attribute types </td></tr>
<tr> <td class="highlighted"  colspan=1 rowspan=1 > X.521 </td><td colspan=1 rowspan=1 > ISO/IEC 9594-7 </td><td class="highlighted"  colspan=1 rowspan=1 > The Directory: Selected object classes </td></tr>
<tr> <td colspan=1 rowspan=1 > X.525 </td><td colspan=1 rowspan=1 > ISO/IEC 9594-9 </td><td colspan=1 rowspan=1 > The Directory: Replication </td></tr>
<tr> <td colspan=1 rowspan=1 > X.530 </td><td colspan=1 rowspan=1 > ISO/IEC 9594-10 </td><td colspan=1 rowspan=1 > The Directory: Use of systems management for administration of the Directory </td></tr>
</table>
Question:
What ITU-T Number is used for Attribute types and object classes? 
Answer:
X.520 and X.521 is for attributes and object classes.