## 3 Results and Discussion
### Answering RQ3 to RQ5
The evolution of display technology is also considered for the analysis of display devices. Starting from the most dominant display device, HMD is mentioned in 50 articles, which is 25% of the selected articles, all of which comprehensively included HMD into their content. This is thanks to the advantages of HMD, which are portability, hands-free \begin{table}
\begin{tabular}{c c} \hline
**Classification Criteria** & **References** \\ \hline \multirow{8}{*}{Software} & [7,51,60,62,65,66,68,71,73,75] \\  & Content design & [84,92,98,100,104,108,109,112,122,129] \\  & [139,141,142,146,150,182,184-186,193] \\  & [197,201,202,205,207,210,212-214,223] \\ \hline \multirow{8}{*}{Software} & [81,152,53,55,55,57,63,64,67,69] \\  & [58,96,172,77,79,81,86,87,90] \\  & [93,95,97,99,103,105,113,119,125-127] \\  & Interaction design & [130,131,140,144,145,147-149] \\  & 95 & [151-163,165-181] \\  & & [183,187–189,192,194,195,198,200,204] \\  & & [206,208,209,211,215-220] \\  & & [221,222,224,225,235] \\ \hline \multicolumn{2}{c}{Agent-based AR} & [88,102,190,203] \\ \hline \multicolumn{2}{c}{Knowledge-based AR} & [30,56,116,121,132,138,143,164] \\  & & [191,196,199,210] \\ \hline \end{tabular}
\end{table} 
Table 12: Articles on Layer 2 Implementation-sublayer Software from 2010–2021. Figure 12: Number of articles about AR software design in manufacturing over years 2010–2021. interaction and user experience enhancement through direct overlaying of computer-generated information onto users' views. The HMDs mentioned in the selected articles are usually commercial devices that are available on the market. Hololens is the one that is utilized the most among commercial optical see-through HMDs (OST-HMD) [8; 52; 53; 220]. According to [240; 241], another type of HMD is video see-through HMD (VST-HMD). In this category of HMD, Samsung Gear VR or Oculus are remarkable candidates. A customized VST-HMD based on the use of Z800 3D visor by Emagin combined with a USB camera--Microsoft LifeCam VX6000--was made to create an interactive AR implementation for virtual assembly in [167]. In terms of technology, the current HMD devices' ergonomics (weight, resolution, field of view (FOV)) have improved compared to the past, and have moved closer to the industrial requirements for long-term implementation. The technology of OST-HMD allows users to observe the real context through a transparent panel while at the same time seeing the computer-generated information projected onto it. The VST-HMD has cameras affixed to the front of the HMD which captures the real-world images, superimposes the digital information onto those images and then displays AR content through a small display area in front of the user's eyes [242]. Due to the amount of information that needs processing, VST-HMDs usually have higher latency (the time gap between the real world's occurring events and the ones perceived by users' eyes). The current challenges of both types of HMD technology are system latency, FoV, costs, ergonomics and view distortion [2]. The second dominant display device is HHD, with 42 articles, or 21% of selected articles for this SLR. HHDs utilized in the articles are mainly commercial devices such as tablets and mobiles. The greatest advantage of using these HHDs is that the users are more familiar with the technology because mobiles and tablets are also used in daily activities such as work, entertainment, etc. In addition, their portability, cross-platform development, cost and capabilities also make them promising alternatives to HMDs [77; 195]. However, in tasks or activities requiring both free hands and intensive manual interaction on the shopfloor, such as assembly [160], quality control [30; 194], etc., HHDs were not an appropriate selection in some cases. The third and fourth trending display devices providing in-situ, hands-free AR displaying content are Monitors/large screens (34 articles, or 7% of the total) and Project for spatial display (19 articles, or 9.5% of the total).