TOPIC: Computed ETC as an input to forecasts

Section 4.4.1.2 and 4.4.1.3 (Schedule and Cost Forecasts respectively) say that these forecasts are derived from the computed time ETC and computed ETC respectively.

Q1: What is computed ETC and computed time ETC?

Further, the PMBOK Guide says "This is typically expressed in terms of schedule variance (SV) and schedule performance index (SPI)", where "This" refers to the forecasts.

Q2:I thought ETC is the forecast! If the ETC is itself a forecast, how can it be used to forecast SV and SPI?

Q3: Aren't SV/SPI and CV/CPI performance and efficiency indicators and not forecasts?