allkeys.txt: An example of a series of collating-table entries as defined by UCA ( Unicode Collation Algorithm).

Actually UCA says allkeys.txt is a “collation element table”, that is, it is the part of a collating table which shows collating elements.

COLLATING ELEMENT: The unit which linguistically-aware users perceive as the minimal building block in string comparisons.

Usually there is a one-to-one relation between characters and collating elements, for example in English there is a character “A” and a collating element for “A”. More rarely there is a many-to-one relation, for example in traditional Spanish the two-character combination “LL” is a single collating element.

Usually there is a one-to-many relation between collating elements and weights (because there are multiple levels); however, for an ignorable character, one collating element has zero weights.

COLLATING TABLE: A table which describes all the rules for a collation, including Posix-like “Locale” declarations and a list of collating elements.

Here are entries for collating elements from two sources, ISO 14651 and allkeys.txt:

[From ISO 14651]
<U0024> <S2C4>;<BASE>;<MIN>;<U0024> % DOLLAR SIGN
<UFF04> <S2C4>;<BASE>;<WIDE>;<UFF04> % FULLWIDTH DOLLAR SIGN
<UFE69> <S2C4>;<BASE>;<SMALL>;<UFE69> % SMALL DOLLAR SIGN

COLLATION ELEMENT: Do not use. Use collating element.

COLLATION TABLE: Do not use. Use collating table.

COLLATING TABLE ENTRY: A line in a collating table, representing one fact.

Each “line” in allkeys.txt (which is a subset of a collating table) is an entry for one collating element.

CONTRACTION: A mapping from N characters to less-than-N collation elements.

Contraction is rare, for example the character “C” has one collation element “C”. But take an example from traditional Spanish: “LL” is a single collation element between “L” and “M”. Contraction also occurs when there has been decomposition. For example here are two collating element entries (from allkeys.txt 5.0.0):

0622  ; [.15E2.0020.0002.0622] # ARABIC LETTER ALEF WITH MADDA ABOVE
0627 0653 ; [.15E2.0020.0002.0622] # ARABIC LETTER ALEF WITH MADDA ABOVE

EXPANSION: A one-to-many mapping from collating element to weighting levels.

For example, German Sharp S may be treated as “ss”, so the allkeys.txt entry for collating element 00DF (Sharp S) is:

00DF  ; [.11AF.0020.0004.00DF][.0000.0199.0004.00DF][.11AF.0020.001F.00DF] # LATIN SMALL LETTER SHARP S;

IGNORABLE CHARACTER: A character which has one collating element which has no significance for comparison. One ignorable character has one collating element, but zero weights at all levels. For example (from allkeys.txt): 0591 ; [.0000.0000.0000.0591] # HEBREW ACCENT ETNAHTA This is ignorable for three levels but not four levels. Therefore it is an “ignorable character” when you produce a weight string for one, two, or three levels. “Ignorable at level 1” means the level-1 weight is ignorable, as represented by 0000 in allkeys.txt. “Fully ignorable” means ignorable for all levels.

ISO 14651: The ISO/IEC 14651 “International String Ordering” standard.

Draft documents:

LEVEL: A prioritization order for weights.

Each level has a name “level + number”, for example “level 1”, “level 2”, “level 3”, “level 4”. (Do not use, or rarely use, equivalent terms “primary”, “secondary”, “tertiary”, “quaternary”.) Typically level 1 is the character-differs level for WHERE clauses, levels 2 and following are case-differs or accent-differs something-minor-differs levels which might be useful for ORDER BY clauses. For example, from allkeys.txt 5.0.0:

0061  ; [.0FD0.0020.0002.0061] # LATIN SMALL LETTER A
24D0  ; [.0FD0.0020.0006.24D0] # CIRCLED LATIN SMALL LETTER A;
0041  ; [.0FD0.0020.0008.0041] # LATIN CAPITAL LETTER A

ORDERING KEY: Do not use. Use “weight string”.

SORTKEY: Do not use. Use “weight string”.

SUBKEY: A sequence of weights for a single level.

UCA: Unicode Collation Algorithm as described in Unicode Technical Standard #10, http://www.unicode.org/reports/tr10.

WEIGHT: A positive numeric value used for comparisons.

Weights come from collating tables and go to weight strings. Often weight appears as a 4-digit number in collating tables. For example (from allkeys.txt):

0062  ; [.0FE6.0020.0002.0062] # LATIN SMALL LETTER B

WEIGHT STRING: A binary string, sometimes called a “sortkey” or an “ordering key”, produced by taking a series of weights from a collating table for a certain number of levels, ordering them by position and level, and outputting.

For example: starting with a character string ABC, and knowing that the number of levels is 2, look up the collating elements for A and B and C in allkeys.txt 5.0.0:

0041  ; [.0FD0.0020.0008.0041] # LATIN CAPITAL LETTER A
0042  ; [.0FE6.0020.0008.0042] # LATIN CAPITAL LETTER B
0043  ; [.0FFE.0020.0008.0043] # LATIN CAPITAL LETTER C

WEIGHTING ELEMENT: A sequence of weights, in ascending order by level.

For example, from allkeys.txt 5.0.0:

00DF  ; [.11AF.0020.0004.00DF][.0000.0199.0004.00DF][.11AF.0020.001F.00DF] # LATIN SMALL LETTER SHARP S

ZERO WEIGHTS: The meaning is “an empty sequence of weights” (the ISO 14651 definition), not “weights with value 0000” (the UCA definition).

For example (from allkeys.txt):

0591  ; [.0000.0000.0000.0591] # HEBREW ACCENT ETNAHTA