Not sure but from reading the export_descr_building file
Its the heavy cav that get a faction wide bonus

heres the listing
building guild_swordsmiths_guild
{
convert_to guild_swordsmiths_guild
levels swordsmiths_guild m_swordsmiths_guild gm_swordsmiths_guild
{
swordsmiths_guild requires factions { northern_european, middle_eastern, eastern_european, greek, southern_european, }
{
convert_to 0
capability
{
weapon_melee_blade 1
}
material wooden
construction 1
cost 1000
settlement_min city
upgrades
{
m_swordsmiths_guild
}
}
m_swordsmiths_guild requires factions { northern_european, middle_eastern, eastern_european, greek, southern_european, }
{
convert_to 1
capability
{
weapon_melee_blade 1
}
faction_capability
{
heavy_cavalry_bonus bonus 1
}
material wooden
construction 1
cost 2000
settlement_min large_city
upgrades
{
gm_swordsmiths_guild
}
}
gm_swordsmiths_guild requires factions { northern_european, middle_eastern, eastern_european, greek, southern_european, }
{
convert_to 2
capability
{
weapon_melee_blade 1
}
faction_capability
{
heavy_cavalry_bonus bonus 1
}
material wooden
construction 1
cost 3000
settlement_min large_city
upgrades
{
}
}
}
plugins
{
}
}