Knowledge of the plywood quality classes will help you to better choose the boards for the planned application and the expectations regarding the final quality of the product.
QUALITY CLASSES OF GENERAL PURPOSE PLYWOOD
We distinguish quality classes from I to III, where class I has the least acceptable defects. The right side can therefore be finished in quality classes I to III, while the left side always corresponds to quality class III.
Class I / III plywood
Class II / III plywood
Class III / III plywood
right side class I
acceptable defects:
healthy and completely overgrown knots (in a limited number)
decayed and ungrown knots and knot holes (up to 6 mm, sporadically)
color-matched inserts (up to 3 items / m2)
minimal color variation
right side class II
acceptable defects:
medullary spots
natural streaks
hard rot
sound knots, fully or partially overgrown
decayed and non-stuck knots and knot holes covered with putty (in a limited number)
color-matched inserts (up to 6 items / m2)
cracks patched with putty
internal and external tabs - sporadically
right side class III
acceptable defects:
natural streaks
medullary spots
hard rot
sound knots, fully grown together
broken knots and knot holes covered
putty (up to 40 mm)
surface roughness
sanding
external and internal tabs
larger cracks, patched with putty
QUALITY CLASSES OF VENEERED PLYWOOD
In class I (right side) the following defects are allowed:
single, fine knots healthy, bright and overgrown
point glue punctures (up to 3 mm).
In class II (left side) the following defects are allowed:
healthy knots, overgrown light (up to 10 mm) and dark (up to 5 mm)
false heartwood (up to 1/3 of the surface)
patched (up to 1 m) and untreated cracks at the edges of the plywood
exact and inaccurate bonding at the edges, outer laps at the edges
solar hauls (1/10 area)
point glue punctures.
JOINERY BOARD QUALITY CLASSES
CLASS I / II
CLASS II / II
In class I (right side) the following defects are allowed:
fused knots and partially fused knots
single knot holes seasoned with an insert of a selected color
narrow cracks (up to 3 mm) covered with putty.
The quality of the left side corresponds to class II.
In class II (right and left side) the following defects are allowed:
English 
Polish 
German