What's mean CEX/DEX/PEX/D-PEX? noBD, noBT?
CEX - Intended for use on retail consoles. Can do most things except homebrew development / debugging / target manager API. Equivalent to Rebug Lite.
DEX - Intended for use on debug consoles, either official (DECH) or retail units converted to debug units. Will not install on unmodified retail units. Has all debugging features. Plays commercial games but lacks certain retail features (e.g., DLNA support). Has a more powerful debug settings menu. PSN connectivity is technically supported but will probably get you banned within minutes.
PEX - Intended for use on retail consoles. Combines CEX and DEX and allows you to switch between them without having to reinstall the firmware. Behaves almost identical to CEX when in CEX mode and almost identical to DEX when in DEX mode.
D-PEX - Identical to PEX but intended for use on debug consoles (official or converted).
noBD - Intended for use on consoles where the Blu-Ray drive is faulty (specifically the logic board, if the laser is broken it's not considered faulty for this purpose). Generally a PS3 with a broken Blu-Ray drive will not launch any games, not even digital ones (.pkg). This variant bypasses this issue.
noBT - Same thing but for use on consoles with broken Bluetooth.
noBD+noBT - For consoles with both Blu-Ray drive and Bluetooth broken.
Unfortunately it is not possible to install the noBD/noBT variants with flash writer at this point - attempting to flash HFW 4.90.1 (which is a requirement for flash writer) on a console with a broken Blu-Ray drive / Bluetooth will brick the console. You will be able to flash these if and when bgtoolset returns, because it patches OFW directly, without having to go through HFW. For the time being those variants are can only be used to update units already on CFW.
* For the sake of completeness - You can technically install the noBD/noBT variants on non-broken consoles (and flash writer will work in that case) but there's no real reason to do so - iirc those variants break backwards-compatibility, maybe some other stuff. If your Blu-Ray and Bluetooth are working then stick with the regular variant.