Can someone please explain this code below

[Former Member]

and also suggest how can i restrict the user from entering special characters in purchase order in this code

<td class="identifier"> <% if ((!ui.isOciTransfer() || !ui.isBasket()) && ui.isElementVisible("order.numberExt")) { %> <label for="poNumber"> <isa:translate key="gs.att.ext.refno" /> </label>

<td class="value"> <% if (!ui.isOciTransfer() || !ui.isBasket() || ui.isElementVisible("order.numberExt")) { %> <% if (ui.isElementEnabled("order.numberExt")) { %> <input type="text" class="textinput-large" maxlength="<%-- PIER 12558334 PO number only 30 chars =(ui.isBackendR3()) ? 20 : 35 --%><%=30%>" name="poNumber" id="poNumber" value="<%= JspUtil.encodeHtml(ui.header.getPurchaseOrderExt()) %>" /> <% } else { %> <%= JspUtil.encodeHtml(ui.header.getPurchaseOrderExt()) %> <% } %> <% } else { %> <input name="poNumber" type="hidden" value="<%= JspUtil.encodeHtml(ui.header.getPurchaseOrderExt()) %>" /> <% } %> --</td>