In terms of how much difference it makes to you, the phone owner, there isn’t a great deal of difference between a SIM unlock code, subsidy code and network unlock code.
The reason for this question is typically somebody is trying to use a SIM card other than that provided by the network operator who provided the phone. Network operators sell their mobile phones at a much reduced rate in order to entice customers into a contract of typically 12 or 18 months. In practice what the operator is doing is adding the cost of the cell phone into the monthly contract charge for the service. This is why they put in place restriction codes to ensure that the mobile phone cannot be used with another carrier while they recoup its initial cost.
You can get a new SIM card on any network for next to nothing from most phone retailers, supermarkets and the internet. To get the unlock code for a HTC Sensation XE, the best thing to do is first ask the people who had it locked in the first place, your network service provider. If you have been a long standing customer of theirs the likelihood is they will offer it to you for free, and may even offer to do the unlock for you in one of their high street stores.