The version you're mentioning, specifically, is a USA release for the PS Vita, with the interesting addition of "NoNpDrm" in the title.
Here's the long story:
The USA release of Ninja Gaiden Sigma Plus for the PS Vita, specifically with the "NoNpDrm" label, indicated that the game had been patched to remove the DRM protection. This meant that players could enjoy the game without any online requirements, making it a more appealing option for those who preferred to play offline. Ninja Gaiden Sigma Plus PS VITA -USA- -NoNpDrm-