In order to make a protein claim (ie: "Good source of protein," "Excellent source of protein," etc.) a Percent Daily Value (% DV) is required to be listed.
In determining the % DV, it works much like this quote we've all heard:
"It's not the years in your life, but the life in your years that counts."
Likewise, it's not just the amount of protein in your food (protein content), but the quality of protein in your food that counts (complete vs. incomplete protein).
Quality versus Quantity.
There is a second stage in meeting a "good" versus "excellent" protein claim. That stage addresses quality versus mere quantity. It is not enough to acknowledge the protein content (a quantity story). PDCAAS (a quality story) must be known in advance of making an eligible protein claim on a food product, specifically in stating a % DV.
This can get confusing FAST, and I don't want to lose you. In understanding the quality story behind protein, here are the terms covered (in order & numbered for easy scrolling/locating):
 Amino acids: [A] Essential and [B] Nonessential
 Complete protein
 Incomplete protein
 Complementary protein
If you merely want to know what to look for in a product when shopping, scroll to “Take-aways” at the very bottom. The content in between is ideal for any one curious about the conversation of protein quality, and is a great introduction to a food scientist new to protein-related claims.
We'll get to PDCAAS in a moment, but first…
Why all the fuss?
A food scientist is responsible for complying with regulations of nutrition labeling, formulating products of high protein quality consistent with claims made, and testing the effects of food processing on protein digestibility - all with the end goal of delivering the nutritional properties expected of the food we consume.
 What is PDCAAS?
An acronym for Protein Digestibility Amino Acid Score.
PDCAAS is one of many methods used to evaluate a food's protein quality. It is mentioned here because it is the FDA-recognized method, and thus, utilized in the current regulations. PDCAAS is used by the food scientist to:
(1) Determine the % DV of a food’s protein source for Nutrition Facts panels.
(2) Formulate in alignment with protein-related claims on food products.
PDCAAS is a numerical evaluation (typically expressed in either decimal or percentage form) of a food's quality of protein. Phrased another way, PDCAAS identifies the protein source's sufficiency in providing certain amino acids at certain amounts (AKA amino acid composition or amino acid profile). More specifically, it focuses on a food product’s essential amino acid composition (discussed below).
The quality of protein works on a scale ranging from 0.0 to 1.0 (converted to percentage form, 0% to 100%). The highest PDCAAS value of any protein is 1.0 (100%). Higher scores suggest foods have a more complete profile of essential amino acids, or a higher protein quality. The limiting amino acid (that with the lowest percentage) determines the protein quality score.
In case you're more curious about the specifics of PDCAAS, I found these resources to be especially helpful:
(1) Merieux Nutrisciences
(2) Suzanne Nielsen's Food Analysis textbook
(3) FDA CFR
(4) Joint Report FAO/WHO, Protein Quality Evaluation
(5) Ostomy Wound Management
 Amino acids (AA).
There are twenty different amino acids (AA) that can be combined to make every type of protein in the body. Amino acids can be classified in a number of ways. When classified by their nutritional roles, they come down to two main categories:
(1) essential amino acids or
(2) nonessential amino acids.
[A] Essential amino acids: (AKA indispensable amino acids, IAA)
AA that are not synthesized by the human body, and must be obtained through one's diet for normal body functioning.
[B] Nonessential amino acids: (AKA dispensable amino acids, DAA)
AA synthesized by the human body from essential amino acids (that are either consumed in food, or in the normal breakdown of body proteins).
In case you're more curious about the specifics of amino acids, I found these resources to be especially helpful:
(1) Suzanne Nielsen's Food Analysis textbook
(2) Joint Report FAO/WHO, Protein Quality Evaluation