"1.boundary - That which indicates or fixes a limit or extent, or marks a bound, as of a territory; a bounding or separating line; a real or imaginary limit.
in its original and strictest sense, is a visible object or mark indicating a limit. Bound is the limit itself. But in ordinary usage the two words are made interchangeable.
2.Constraint - implies strong binding force; as, the constraint of necessity; the constraint of fear. Compulsion implies the exertion of some urgent impelling force; as, driven by compulsion. The former prevents us from acting agreeably to our wishes; the latter forces us to act contrary to our will. Compulsion is always produced by some active agent; a constraint may be laid upon us by the forms of civil society, or by other outward circumstances.
i think boundary is more bigger than constraints. because when we mention about constraint, it is doing in boundary. in other word, boundary can include many constraints.
3.interfaces -a surface forming a common boundary between two things.
4.interrelations- A logical or natural association between two or more things: connection, correlation, interconnection, interdependence, link, linkage, relation, relationship, tie-in. Informal hookup. See connect.
interrelations is more bigger than interfaces. because interfaces can connect only 2parts while interrelations include more than 2parts.

example) boundary can be one country which allows people to move. constraints are law and promises which restrcits people's activites.
interface can be 2departments which have different task but same goal. that's why they are related closely. but interrelation is company which include all departments.
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