In multi_cfg.py, why num_class=1 for extra_head ?

[praj441]

The model configs are mentioned as follow in file config/multi_cfg.py :
model = dict(
type='SingleStageDetector',
backbone=dict(
type='SimpleVoxel',
num_input_features=4,
use_norm=True,
num_filters=[32, 64],
with_distance=False),

neck=dict(
    type='SpMiddleFHD',
    output_shape=[150,440,440],
    num_input_features=4,
    num_hidden_features=64 * 19,
),
bbox_head=dict(
    type='SSDRotateHead',
    num_class=9,
    num_output_filters=256,
    num_anchor_per_loc=2,
    use_sigmoid_cls=True,
    encode_rad_error_by_sin=True,
    use_direction_classifier=True,
    box_code_size=7,
),
extra_head=dict(
    type='PSWarpHead',
    grid_offsets = (0., 2.), #Almost half of the point cloud range
    featmap_stride=.4,
    in_channels=256,
    num_class=1,                                     
    num_parts=28,
)

)

Why num_class=1 in extra_head session, however, this config file is for multiple classes?

[nmll]

why the featmap_stride=0.4？