if we want to add custom fields like textbox,select box to existing system -> configuration tab -> catalog.
<?xml version="1.0"?>
<config>
<sections>
<catalog>
<groups>
<frontend>
<fields>
<mayankpatel translate="label">
<label>Custom Textbox Value</label>
<frontend_type>text</frontend_type>
<source_model>adminhtml/system_config_source_yesno</source_model>
<sort_order>1</sort_order>
<show_in_default>1</show_in_default>
<show_in_website>1</show_in_website>
<show_in_store>1</show_in_store>
</mayankpatel>
</fields>
</frontend>
</groups>
</catalog>
</sections>
</config>
<?xml version="1.0"?>
<config>
<sections>
<catalog>
<groups>
<frontend>
<fields>
<mayankpatel translate="label">
<label>Custom Textbox Value</label>
<frontend_type>text</frontend_type>
<source_model>adminhtml/system_config_source_yesno</source_model>
<sort_order>1</sort_order>
<show_in_default>1</show_in_default>
<show_in_website>1</show_in_website>
<show_in_store>1</show_in_store>
</mayankpatel>
</fields>
</frontend>
</groups>
</catalog>
</sections>
</config>