An assembly is not defined only by its final configuration. Parts must be brought into place through a physically feasible sequence.

A precedence relation states that one operation must occur before another. For example, an internal seal may need to be installed before a cover blocks access.

Assembly planning therefore checks:

• insertion direction and collision-free access;
• tool access for fastening or joining;
• temporary stability before all parts are fixed;
• precedence created by enclosed components;
• opportunities to build independent subassemblies;
• inspection or test points that should occur before later operations hide them.

A precedence diagram can be represented as a directed acyclic graph. If operations $A$ and $B$ must both precede $C$, but are independent of one another, the graph contains

A ─┐
   ├─> C
B ─┘

This means $A$ and $B$ can potentially be performed in either order or in parallel.

Sequence planning matters in design. A fastener that is easy to model but inaccessible after another part is installed creates a manufacturing problem even if the final CAD assembly has no interference.

The best assembly sequence is therefore constrained by geometry and process requirements but can still be optimized for handling, parallel work, tooling and verification.