Alk Phos has a couple of isozymes ( versions of the molecule) which are produced in different areas. Liver and bone are 2 major areas they are used for. When bone is actively being destroyed and / produced by normal or abnormal means, the enzyme levels go up accordingly. The docs need to figure out with other evidence if they are dealing with bone mets and then use the AlkP as one tool.
Normal lab values will indicate average adult "resting" metabolism of bone and liver. If either / both areas are affected the AlkP may be up so they'll check isozyme, if it is bone they'll do different scans which we have not talked about here but any of CT, MRI, bone scan, PET could be used.
growing children/teens, post surgery bone fracture patients, post liver resection patients etc. may also have elevated AlkP as that enzyme will be active during repair and maintanence of the healing state as well as during ongoing "damage" from any of a number of causes
( seems to me I recall the placenta also produces higher levels of one AlkP isozyme … sorry just going from memory here