String - The un-encoded ASCII representation of the URL.

URLDecode is a string class function that converts the URL-encoded string in ASrc to its representation in the US-ASCII character set. URLDecode is based on the URL character encoding rules as described in the Internet Standards document: 

RFC 1738 - Uniform Resource Locators (URL)  

URLDecode can used to reverse any string encoding performed using URLEncode. 

URLDecode converts all occurrances of the '+' character to ' ' (Space) (Decimal 32). URLDecode reads any values represented as a hexadecimal character triplet consisting of the "%" character followed by the two-digit hexadecimal value for the octet, and returns the value to its US-ASCII representation.

TIdURI.URLEncode