Base64 Decode tool is designed for reverse process of Base64 encoding. So this tool will only convert base64 encode data to original string, otherwise will through an error.
Base64 is a cluster of similar binary-to-text encoding schemes that represent binary data in an ascii string format by translating it into a radix-64 representation. So it is not an ascii encoding method. Base64 encoding represents a string in a binary format and converts it into simple text string.