[ericbsmith]

Q: When adding traits (especially spells) a selection window opens asking me to choose between possible prerequisite chains; some of the prerequisite chains start with a prefix such as ST: or SK: - What do those prefixes mean?

Those prefix tags tell the program what type of trait it's looking for, differentiating between traits types such as Advantages and Skills. The full list of prefix tags is as follows:

ST: or AT: Statistic/Attributes (synonymous)
AD: Advantages
PE: Perks
PO: Powers (GURPS 3rd Edition compatibility, not used in GURPS 4th Edition. Powers & Perks share the same tab in GCA, so PE: and PO: are effectively synonymous)
DI: Disadvantages
QU: Quirks
TE: or PA: Template/Package (synonymous)
GR: Groups (as defined under the [GROUPS] header)
SK: Skills
SP: Spells
EQ: Equipment
LI: Lists

SAMECAT: refers to an item from the specified category, of the same type as the item being needs checked.
CA: or ADCAT: refers to an item from the specified category, for advantages
DICAT: or DISADCAT: refers to an item from the specified category, for disadvantages
CL:, SKCAT: or SKILLCAT: refers to an item from the specified category (class), for skills
CO:, SPCAT: or SPELLCAT: refers to an item from the specified category (College), for spells
EQCAT: or EQUIPCAT: refers to an item from the specified category, for equipment
TECAT: or PACAT: refers to an item from the specified category, for templates/packages