You should only accept a check for your injuries when you know the full extent of your injuries. That's because some people think that they can go to the insurance company, get some money for some injuries, and when they realize they maybe have some injuries connected to the accident, they can request more funds. However, it doesn't work that way.
When you settle your case, you settle it for all injuries in connection with the case. So, when you accept that initial check, they will usually require you to sign a settlement release releasing all other claims. You should only settle your case when you know all of your injuries and are going to be fully compensated for all of your injuries.