Summer Variation is the 3x3x3 subset for orienting the last layer corners while solving the R U R' case for the F2L last slot. This set of algorithms could be learned as an addition to Winter Variation.
It can be hard to distinguish between different Winter/Summer cases. They all look very similar. One way to identify each individual case when you are learning is to assign a number to each corner and remember which string of numbers corresponds to which algorithm. This is slow when starting, but eventually you wont think of the numbers anymore. The three number string in front of each algorithm refers to UFR, UBR,and UBL respectively, for winter variation and UBR,UBL, and UFL respectively for summer variation. The s and w after the three numbers means it is either summer or winter variation. A zero means that corner is correctly oriented, a 1 means one turn CCW and a 2 means one turn CW.
For example, after doing (R U R') we get case 100w. The first corner is turned CCW, 1, the other two are correctly oriented, 0 and 0, and it is a winter variation case. Another example, after doing (R U R' U R U R') we get case 012s. the first corner, in UBR, is correctly oriented, 0, the second is turned CCW, 1, and the last one is turned CW, 2, and it is a summer variation case. Conversely, a third example, case 102s means this is a summer variation, the "s" after it, and the first corner, in UBR, is turned CCW, the second, in UBL, is oriented, and the third in UFL is turned CW.
000s (L' U2) (R U R') (U2 L) (U' R U R')
001s (RUR') (L U' R' U L' U' R)
002s (M x) D (L U L') D' (M' x')
010s U2 (R L) (U'R')(U L')
011s l F (U' R' D R U R') D' x
021s U2 Mx (D' F2 D) M'X'
022s F2 (r' U'r) F2 (r'U r) (R U R')
100s (RUR') U' (R U R'U R U2 R')
101s xM (UR'U'L)
102s U R2 (L' U' L) U' F2 (R' F2) (U2 R2)
110s U2 (L U L' U') (F2 L F2 L2)
111s U2 R (L U') R2 (U L' U' R)
112s (R U2) (R' U R U')(R'U R U)(U R')
120s (R U' R)(D R' U2) (R D' R2)
121s (R U R) (R U2 R U R' U R)
122s (R U2 R)(D R' U' R D' R2)
200s (l F) (U L U'R' U L U') x
201s (R U'R'U'R U R' U'R U' R')
202s (R U' R2 U' R2 U' R2 U2 R)
210s (R U) (R2 U' R U' R' U2 R)
211s (R U' R' U' R U2 R2 U'R2 U'R2 U2 R)
212s (R U R') U (R' U' R U' R' U2 R)
220s (l F l2) (U' L'U R U' L U) x'
221s (U' R U) R (D R' U) (R D' R2)
222s (R' U r B'l RU R'L'
- Speedsolving.com: COLS/"Summer Variation"/set of algs without a name