There are two types of KNOX Security implementation within samsung devices. 1. Hardware level 2. API level. All samsung flagship devices including A series comes with fully fledged hardware level KNOX security. whereas the budget segment especially M series comes with Software based or API level KNOX security integration. thats the reason you dont see samsung pay, goodlock and other premium security features as compared to SM flagships.
M51 has Knox along with M31s. There is nothing called software only KNOX. API is not what you think. Knox API is used to called Phone's Knox firmware (knox chip and related components) to do task. Let me give you example. Consider Smartphone camera. Could you say your phone has software only camera? When a application needs to access camera related function it uses CameraAPI3 to call camera function, which in turn give access to camera hardware of phone, similar thing happens in Knox scenario. Now that Knox API is also available in non Knox phone because samsung uses same software stack independent of knox, if certain phone doesn not support knox then the service just return result as false. Hope it helps.
