**4. Conclusion and future directions**

Accretion is an essential process to drive black hole growth, and it is thought to work in different types of accreting black hole systems from stellar-mass galactic

black holes to supermassive black holes located in the centers of galaxies. Now, we are near to reaching a consensus that the physics in controlling the accretion and the associated ejecting process is exactly same in various kinds of accreting systems. In this chapter, we explored the correlations concerning the universal evolution among various accreting systems: (1) the correlation among X-ray variability, the black hole mass, and bolometric luminosity; (2) the fundamental plane of black hole activity, i.e., the correlation among core X-ray and radio luminosity and black hole mass; (3) the inverse correlation between radio loudness and Eddington ratio; (4) the correlation between X-ray loudness and Eddington ratio. These evidences ensure us to apply the theory to study the accretion process in, e.g., high red-shift quasars, the evolutionary connection between FRI and FRII radio galaxies, and the accretion signatures of low luminosity AGNs, and so on. However, the unified models for black hole accretions still face challenges in a few types of cases in practical applications: (1) the extremely high and super-Eddington accreting systems are poorly understood in both XRBs and AGNs; (2) the intermediate region between stellar-mass black holes and supermassive black holes in the fundamental plane of black hole activity is still unfilled; (3) the intermediate-mass black holes are absent; (4) the lack of evidence of state transition in individual AGNs still throws doubt on the unified scheme in AGNs. Furthermore, the unified models for black hole accretion have weak constraints and well understanding of the ejection process. Especially, in radio-quiet AGNs, corona and windlike outflows are the two primary radio-emitters except for the jets, while it is unclear how the three processes interplay with each other and which one is in holding the dominance with the accretion flow evolves. Future high-resolution observations are essential to identify the radio origin. Additionally, it may shed light on how jet bases connect with accretion disk and how jet forms. Again, the high-resolution radio observations of intermediate-mass black holes and super-Eddington AGNs are equally important in filling the break between XRBs and AGNs in the fundamental plane of black hole activity and extending it to the super-Eddington regime.
