Well… HTTP proxy can run in both explicit and transparent mode and this has nothing to do with SSL-Bump.
These are 2 different aspects and the only relationship is when you want/need to deal with HTTPS in transparent mode or if you want/need to analyse HTTPS content.
However, what needs to be understood is that from browser view point, proxy is either defined (whatever the way you define it) or not. If proxy is defined, then this is an explicit proxy. If not, this is transparent proxy. Which means that if proxy side, one can have both in parallel, client side (browser) you have only 3 choices with no overlap:
explicit proxy
no proxy -> with transparent proxy intercepting
no proxy... without proxy ;D
If SSL-bump is configured (whatever proxy mode, explicit or transparent), trusting certificate generated to intercept HTTPS flow is mandatory to prevent warning messages.
This often means to deploy, client side, CA public key to be trusted.
Keep in mind we are only discussing technical aspects here, not all the legal aspects with HTTPS flow being broken and intercepted.
While designing your proxy, you have to determine whether your goal is to filter access (e.g. prevent facebook access) which doesn't require any SSL-bump, even for HTTPS, but does require explicit proxy or if you need/want to intercept HTTPS, which means SSL-Bump thus certificate.
