Interaction of buckling modes

I’m reading this paper on structural members subject to both bending and compression.

In the first paragraph, in context of the equations 6.61 and 6.62, it is said:

The second two terms are modified by factors
that allow for the interaction between the different modes of
buckling.

What is meant by the interaction between different modes of buckling? How do buckling modes "interact"? I tried searching online but couldn’t really find a good source. Checking a member against buckling I’ve understood that only the first mode is usually checked, with $N=1$ in Euler buckling equation.

$$begin{align}
frac{N_{Ed}}{N_{b, y, Rd}} + k_{yy}frac{M_{y, Ed}}{M_{b, Rd}} + k_{yz}frac{M_{z, Ed}}{M_{c, z, Rd}} &leq 1
frac{N_{Ed}}{N_{b, z, Rd}} + k_{zy}frac{M_{y, Ed}}{M_{b, Rd}} + k_{zz}frac{M_{z, Ed}}{M_{c, z, Rd}} &leq 1
end{align}$$