SubQ means in the Subcutaneous layer of the skin wich is between the skin and the muscle, or in other words not very far in, IM means intramuscular and it means stickin that bitch in as deep as she will go, usually a 1.5 inch needle (other will say a 1 inch will be fine) best in the Gluteus Medias, or high on hip just below the belt line. Both are used to delay introduction into the system thats why you don't shoot it in your vien like Heroin.
A picture is worth a 1000 words. See the diagram below. Also, If you can reach around that far, right in the Gluteus Medias works very well, easier if some one else sticks ya there though.