It depends on the synthesis mode: one-hot or etc. If onehot has been selected, "none" is synthesized to -----1. Otherwise it will synthesize to 0, "iterative" to 1 and etc. You also can specify the value of them, i.e. (none=0, ...).
Synthesis mode is one of switches in the synthesis tools and you as a designer can change it. For example in Leonardo, after selecting Tools --> Synthesis Wizard --> Next, you will see "encoding Style" when you read input files. Its default value is "Auto" which can be set to "Binary", "Onehot", "Twohot", "Random" and "Gray".