1. Denoted by a solid square
2. if present, they point TO a local mechanism (say T). T is expected to differ between the environment of the experiment and the environment of application.
3. Absence of a selection node implies that that mechanism is expected to stay the same in both environments.

University of Michigan - Ann Arbor

Some useful definitions to better understand the paper.
1. Selection Nodes
2. Functional Constraints
3. Transportability
4. Monotonicity

Helpful Definitions for the Paper

Cinelli, Carlos, and Judea Pearl. “Generalizing Experimental Results by Leveraging Knowledge of Mechanisms.” Technical Report - UCLA, 10 Dec. 2019.

https://ftp.cs.ucla.edu/pub/stat_ser/r492.pdf

Generalizing Experimental Results by
Leveraging Knowledge of Mechanisms

Selection Nodes

Used to determine if a causal relationship can be transported from one environment to another. Determined using d-separation. 
If there are a set of d measures that separate the source of the disparity from the quantity we are estimating i.e. target, then we can say that the quantity is transportable.

Transportable

A functional constraint is a property that is obtained from the nature of the problem under consideration, that must be satisfied.

Functional Constraints

https://en.wikipedia.org/wiki/Constraint_(mathematics)

Reference : Functional constraints

https://en.wikipedia.org/wiki/Monotonic_function

Reference : Monotonicity

Property of a function by which they are either constantly increasing or decreasing i.e. sign of the second derivative of the function always stays the same.

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