A criminal charge for assault can change depending on the nature of the situation and the injuries sustained. One important factor the prosecutor will consider is if there was a serious bodily injury.
According to the UNC North Carolina Criminal Law blog, serious bodily injury is that which causes permanent disfigurement or brings a risk of death. It may also cause extreme pain or permanent loss of a body part or organ. It may also result in a lengthy hospital stay.
Interpretation
The definition often has different interpretations by juries and courts. For example, a jury found in one case that a rather severe injury that causes a lot of pain during the assault was not a serious bodily injury because the pain did not last and did not cause a permanent injury.
Other charges
Often, when a jury or court must decide if an injury is a serious bodily injury, there is another factor that could influence their decision. For example, a case may involve two charges that are similar but have different standards, such as child abuse inflicting serious bodily injury and child abuse suffering serious physical injury.
Having two options may lead a jury or court to rule for the lesser if there are no permanent physical issues from the incident. Whereas, if there is only one charge at hand, the jury or court may feel more inclined to be lenient in the definition of serious bodily injury and more likely to define it as such.
In any situation, though, the definition seems too open to interpretation, so you never know how a jury or court could look at your case and what the outcome might be.