'active' means that the FGT will negotiate the LACP bundle, 'passive' means, the other side will do it. You can use 'active'/'active' or 'active'/'passive', I've used both with success.
The third value is 'static' in which there is no negotiation at all. Might be required if the switch doesn't support LACP. The downside is that the trunk will not be re-configured if one of it's members fails, thus leading to packet loss.
'slow' and 'fast' relates to the number of 'hello' packets sent between the trunk ends. 'slow' is like 2 packets per minute, 'fast' like one packet per second. Some switch vendors expect slow, some fast.
Speed and duplex settings are left on 'auto' nearly 99% of the time. Again, if the other side doesn't properly negotiate the line parameters (speed, half/full duplex) then you have the means to configure them manually. Rarely used.
To check your LACP trunk use 'diag netlink aggregate name your_aggregate_link
'. See this KB article for troubleshooting LACP trunks: https://kb.fortinet.com/k....do?externalId=FD30542