Drops Yarn Groups
Drops yarn comes in different weights or categories.
A group is equivalent to 4ply
B group is equivalent to Double Knitting
C group is equivalent to Aran yarn
D group is equivalent to Chunky
E group is equivalent to Super bulky
F group is equivalent to super bulky or jumbo
Any yarn from the same group can be substituted for another in a pattern. However the lengths may differ so when substituting always calculate the number of yards needed, in order to know the amount of yarn you need.
Here's an example of how to calculate the amount of an alternative yarn needed for a pattern:
The pattern requires 300g of yarn X.
Yarn X runs 170m per 50g, which means 6 balls (of yarn X) x 170 m = 1020 m of yarn needed.
You want to use yarn Y. This yarn runs 150m per 50g.
1020m / 150m = 6.8 = 7 balls of yarn Y needed to replace yarn X.
You can also try working with yarn from a different Yarn Group on your pattern by replacing the yarns as follows:

2 strands A = 1 strand C

3 strands A = 1 strand D

4 strands A = 1 strand E

8 strands A = 1 strand F

2 strands B = 1 strand D

2 strands C = 1 strand E

4 strands C = 1 strand F

2 strands D = 1 strand F

2 strands E = 1 strand F