Hi sukinoz
To change it edit this file/plugins/system/t3/base-bs3/params/template.xml
At the end of file you can find :
<fieldset name="injection_params" label="T3_INJECTION_LABEL" description="T3_INJECTION_DESC">
<field name="snippet_open_head" type="textarea"
class="t3-admin-textarea"
global="1"
filter="raw"
default=""
label="T3_INJECTION_OPEN_HEAD_LABEL"
description="T3_INJECTION_OPEN_HEAD_DESC"
/>
<field name="snippet_close_head" type="textarea"
class="t3-admin-textarea"
global="1"
filter="raw"
default=""
label="T3_INJECTION_CLOSE_HEAD_LABEL"
description="T3_INJECTION_CLOSE_HEAD_DESC"
/>
<field name="snippet_open_body" type="textarea"
class="t3-admin-textarea"
global="1"
filter="raw"
default=""
label="T3_INJECTION_OPEN_BODY_LABEL"
description="T3_INJECTION_OPEN_BODY_DESC"
/>
<field name="snippet_close_body" type="textarea"
class="t3-admin-textarea"
global="1"
filter="raw"
default=""
label="T3_INJECTION_CLOSE_BODY_LABEL"
description="T3_INJECTION_CLOSE_BODY_DESC"
/>
<field name="snippet_debug" type="radio" default="0"
global="1"
label="T3_INJECTION_DEBUG_LABEL"
description="T3_INJECTION_DEBUG_DESC">
<option value="0" class="off">JNO</option>
<option value="1" class="on">JYES</option>
</field>
</fieldset>
Change global="1" to global="0" for the textfield snippet you want to isolate
Best regards