how to associate bias frames to lights / flats

A probably stupid question ...

I'm trying to implement bias frames on top of light/dark / flats

I've shot lights and flats with different gains

So I have a set of bias frames according to light and "lightflats" images

and a set of bias frames associated to flat and darkflats

I don't know, in the initial file selection and loading process, how to associate bias with the regarding images, since i'm asked to select channels/ sessions rather than image type (light /flat)

Where am I wrong ?

clear skies

PS image collection detail (not sure it is useful) just in case

lights :

10 Halpha lights @gain 450 temp1 mapped to session 1

15 darks at gain 450 temp1 (same duration as lights above) mapped to session 1

then

5 Halpha lights @ gain 450 temp2 mapped to session 2

10 OIII lights @ gain 450 temp2 mapped to session 2

10 SII lights @ gain 450 temp2 mapped to session 2

15 darks @ gain 450 temp2 mapped to session 2

for each channel

15 flats + 15 darkflats per channel @ gain 120

Halpha frames mapped to session 1 & 2

OIII et SII frames mapped to session 2

I did 50 bias at gain 450 and (block 1)

50 bias at gain 120 .. (block2)

but I don't know how to associate block 1 to lights and block 2 to flats (if it's possible)