A deliberate change to the planned design or the plan for making, made for a stated reason and recorded with its effect on the design specification.
A design modification is a deliberate change to the design or the plan for making, chosen for a reason you can put into words.
A mistake is an unplanned result, like a leg cut 2 mm short, and it only turns into a modification once you decide what to do about it.
Both belong in the folder, but a mistake gets a correction while a modification gets a justification.
Criterion C looks at how you followed and adapted your plan, and Evaluating asks you to judge the result, so a well argued change is worth writing up for both.
Changing the design and saying nothing turns a defensible decision into something that reads as carelessness.
Four Reasons Designs Legitimately Change
The material behaves differently from what the drawing assumed, such as 3 mm cast acrylic cracking along a line bend that 3 mm extruded acrylic survives.
The machine cannot make what was drawn, like a 3D printer that will not bridge a 60 mm unsupported span without sagging.
A test with the client changes the requirement, for example the client asking for the handle to sit 20 mm lower after holding the first version.
Time or money forces a reduction, so the five drawer units in the drawing become three built properly.
Naming which of these four applies gets you halfway through the justification before you write a full sentence.
Justification: Reason, Evidence, Effect
The reason states what forced the change, in one sentence that contains an actual cause.
The evidence is what you observed or measured, such as "the 3 mm shelf sagged 6 mm in the middle under a 2 kg load".
The effect says what the change does to the design specification, taken point by point rather than in general.
Written in that order the whole entry usually fits in 60 words, which is short enough that you will actually write it.
Attach a photograph or a screenshot to the evidence, because a measurement with an image behind it is much harder to argue with.
Example
Reason: the 3 mm acrylic cracked along the bend line on both of my two attempts.
Evidence: both test pieces split within about five seconds of bending on the line bender, and I kept and photographed them.
Effect: switching to 3 mm polypropylene meets specification point 2, that the clip must flex without breaking, but loses specification point 5, that the body must be transparent.
The client accepted the trade because grip mattered more to her than seeing the contents.
Check the Change Against Every Specification Point
Go back to the design specification and read all of it, not only the point you assume is affected.
Mark each point as still met, partly met or no longer met once the change is in.
A change that improves one point while breaking another is a trade-off, and naming it as one is far stronger than pretending nothing was lost.
Where a point is no longer met, decide openly whether to accept the loss or to change something else to win it back.
Only rewrite the specification itself if the client agrees, because the specification is a promise made to them rather than a note to yourself.
Common Mistake
A change that quietly deletes a specification point is the hardest thing to defend when you come to evaluate the solution.
Writing "I changed it because it looked better" gives a reader nothing to judge.
Blaming a change on time without saying what was cut and why reads as an excuse rather than a decision.
Where the Justification Lives in Your Folder
Short justifications sit inside the making log entry for the lesson in which the change happened.
Gather the significant changes into one modifications table with columns for step, change, reason and effect on the specification.
Show before and after, such as the original CAD drawing beside the revised one with the altered dimension circled in red.
Make the log entry, the revised drawing and the specification check refer to each other, so a reader can move between the three.
Keep the superseded drawing instead of deleting it, since the comparison between the two versions is the evidence.
Exam technique
Three changes explained properly show more thinking than twelve one line notes about paint colour.
Quote the client where you can, because "the lid was too stiff for me to open one handed" is specific in a way that paraphrase is not.
Put the modifications table where the making evidence is, so nobody has to search the folder for it.
Digital Changes Need the Same Three Parts
"Moved the menu" is not a justification until it carries a reason, evidence and an effect on the specification.
User testing gives you usable evidence, such as four out of five testers failing to find the menu when it sat in the sidebar.
Take the before and after screenshots at the same window size, or the comparison shows your browser rather than your change.
Version notes written at each save hand you the timeline of changes for nothing.
Performance counts as evidence too, like a page dropping to a 1.2 second load once the images were compressed from 4 MB to 600 kB.
Active recall
What turns a mistake into a design modification?
Name the three parts of a written justification.
Why check the change against every specification point rather than one?
What is a trade-off, and why is admitting one stronger than hiding it?
Give one piece of evidence you could use to justify moving a menu on a website.