In which table should I save the value of system_settings_form in Drupal 7?
If you use system_settings_form function your data will be saved in 'variable' table.
To get saved data you may use variable_get function.

To show saved value in form

       function mycustom_form($form, &amp;$form_state){
         $form = array();
         $form['position'] = array(
           '#type' =&gt; 'textfield',
           '#title' =&gt; t('Position for the magnifier'),
           '#options' =&gt; array('left' =&gt; 'Left', 'right' =&gt; 'Right','up'=&gt;'Up','down'=&gt;'Down'),
           '#default_value' =&gt; variable_get('position', ''),
           '#required' =&gt; TRUE,
         );

         return system_settings_form($form);
       }


If you want to use saved data from your form in the javascript, you may use next code

drupal_add_js(array('myModule' =&gt; array('position' =&gt; variable_get('position', ''))), 'setting');


Then this data will be available in javascript in Drupal.settings.myModule.position variable