SRB_VRS12 height - EPSG:8691

WKT

VERTCS["SRB_VRS12_height", 
	VDATUM["Serbian_Vertical_Reference_System_2012"], 
	PARAMETER["Vertical_Shift", 0.0], 
	PARAMETER["Direction", 1.0], 
	UNIT["Meter", 1.0]]

WKT2

VERTCRS["SRB_VRS12_height", 
	VDATUM["Serbian_Vertical_Reference_System_2012"], 
	CS[vertical, 1], 
	AXIS["Gravity-related height (H)", up, 
		LENGTHUNIT["Meter", 1.0]], 
	ID["EPSG", "8691"]]

Data from @esri/proj-codes

{
  "wkid": 8691,
  "latestWkid": 8691,
  "macro": "PE_VCS_SRB_VRS12_HT",
  "name": "SRB_VRS12_height",
  "wkt": "VERTCS[\"SRB_VRS12_height\",VDATUM[\"Serbian_Vertical_Reference_System_2012\"],PARAMETER[\"Vertical_Shift\",0.0],PARAMETER[\"Direction\",1.0],UNIT[\"Meter\",1.0]]",
  "wkt2": "VERTCRS[\"SRB_VRS12_height\",VDATUM[\"Serbian_Vertical_Reference_System_2012\"],CS[vertical,1],AXIS[\"Gravity-related height (H)\",up,LENGTHUNIT[\"Meter\",1.0]],ID[\"EPSG\",\"8691\"]]",
  "description": "SRB_VRS12 height",
  "authority": "EPSG",
  "version": "9.5(10.8.0)",
  "deprecated": "no",
  "areaname": "Serbia",
  "extent": {
    "slat": 42.23,
    "nlat": 46.19,
    "llon": 18.81,
    "rlon": 23.01
  }
}