Base Period and speedwork

The concept of periodization is simple: it states to transition from general training (that none-specific to the target event) to specific training (the training 100% specific to the demands of the target event), there is no classic periodization, reverse periodization, etc. Friel interpretation of that concept was borrowed from Tudor Bompa which in turn borrowed it from Russian scientists Matveyev, Tschiene and Verhoshansky.

In terms of what kind of training to do when transition from general to specific training will depend on yor current fitness levels, limiters/needs, time availability and type of main event. There is no specific reason to limit your training load (volume/duration + intensity) beyong doing specific training. Hence during the general phase is an ideal time to target limiters and weaknesses. You can call each cycle anyway you want (use Friels) but the concept is the same, to transition from none-specific to specific training.

i.e. if your main goal is to do an IM then steady state efforts (around 70-80% of your threshold power pace or what Friel calls z2) will be specific to your race; following periodization it means to do those during the specific phase or somewhere between < 12 week prior the race. Before that you can target important adaptations relevant for optimal endurance fitness but not necessarily specific for IM racing such as Tempo (80-95% of threshold (z3)), Threshold (95-105% of threshold (z4)) and VO2 max pace/power (105% > of threshold (z5)). IOW any training done before the 12 week specific phase the athlete could do a mix of this training sessions

OTOH someone training for Olys then Threhold would be specific training and the rest training levels would fall as general training.

Depending what are you training for determine what specific training should be and based on that and limiters/weaknesses you can then determine what stuff you can address during the general phase or what Friel refers as "base phase".

PS. speedwork and intervals are one of those things that mean something different to different people and more often than misunderstood. Any session break down in a set and a recovery interval are defined as 'intervals'; intensity can be high, moderate or low. Speed work is usaully refer as sessions target speeds of VO2 max or higher. Doing some sessions @ VO2 max could be appropiate for certain athletes but in general speed sessions are not very helpful for 99% of triathletes.